U.S. patent application number 11/045088 was filed with the patent office on 2010-01-28 for phase correction element and optical head device.
This patent application is currently assigned to ASAHI GLASS COMPANY LIMITED. Invention is credited to Masao Miyamura, Masahiro Murakawa, Yoshiharu Ooi, Hiromasa Sato.
Application Number | 20100020671 11/045088 |
Document ID | / |
Family ID | 31192427 |
Filed Date | 2010-01-28 |
United States Patent
Application |
20100020671 |
Kind Code |
A9 |
Ooi; Yoshiharu ; et
al. |
January 28, 2010 |
PHASE CORRECTION ELEMENT AND OPTICAL HEAD DEVICE
Abstract
The present invention provides a phase correction element which
can be used for recording and/or reproducing an information of
three types of optical disks for HD, DVD and CD by employing a
single objective lens for HD, and an optical head device. The phase
correction element 100 of present invention comprises a first phase
correction layer 10A formed in a region of numerical aperture
NA.sub.2, and a first phase plate 30A integrally formed; the first
phase correction layer 10A comprising a concavo-convex portion
having a rotational symmetry with respect to the optical axis of
incident light and having a cross-sectional shape of a
saw-tooth-form or a saw-tooth-form whose convex portions are each
approximated by a step form; the first phase plate 30A generating a
birefringent phase difference of about an odd number times of
.pi./2 for linearly polarized light having a wavelength of
.lamda..sub.1; and the phase correction element 100 having a
function of not changing a transmitted wavefront of the wavelength
of .lamda..sub.1 and changing a transmitted wavefront of the
wavelength of .lamda..sub.2 or transmitted wavefront of both
wavelengths of .lamda..sub.2 and .lamda..sub.3 when three types of
incident light in a .lamda..sub.1=410 nm wavelength band, a
.lamda..sub.2=650 nm wavelength band and a .lamda..sub.3=780 nm
wavelength band respectively, are incident.
Inventors: |
Ooi; Yoshiharu;
(Koriyama-shi, JP) ; Murakawa; Masahiro;
(Yokohama-shi, JP) ; Sato; Hiromasa;
(Koriyama-shi, JP) ; Miyamura; Masao;
(Koriyama-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, L.L.P.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
ASAHI GLASS COMPANY LIMITED
Tokyo
JP
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20050226122 A1 |
October 13, 2005 |
|
|
Family ID: |
31192427 |
Appl. No.: |
11/045088 |
Filed: |
January 31, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP03/09746 |
Jul 31, 2003 |
|
|
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11045088 |
Jan 31, 2005 |
|
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Current U.S.
Class: |
369/112.05 |
Current CPC
Class: |
G11B 7/13922 20130101;
G02B 27/0037 20130101; G11B 2007/0006 20130101; G11B 7/1275
20130101; G02B 27/4238 20130101; G11B 7/1353 20130101; G02B 27/4261
20130101; G11B 7/1367 20130101 |
Class at
Publication: |
369/112.05 |
International
Class: |
G11B 7/135 20060101
G11B007/135 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 31, 2002 |
JP |
2002-223085 |
Aug 28, 2002 |
JP |
2002-248835 |
Aug 29, 2002 |
JP |
2002-251911 |
Oct 9, 2002 |
JP |
2002-295731 |
Dec 24, 2002 |
JP |
2002-372435 |
Claims
1. A phase correction element for transmitting three kinds of light
beams having wavelengths .lamda..sub.1, .lamda..sub.2 and
.lamda..sub.3 respectively
(.lamda..sub.1<.lamda..sub.2<.lamda..sub.3), which has an
area of numerical aperture NA.sub.2 and an area of numerical
aperture NA.sub.1 including the area of numerical aperture NA.sub.2
(NA.sub.1>NA.sub.2) in an element plane in which the light is
incident, of the phase correction element; wherein in the area of
numerical aperture NA.sub.2, and a first phase correction layer
comprising a concavo-convex portion of a saw-tooth-form or an
approximated saw-tooth-form having a cross-sectional shape of a
saw-tooth-form or a saw-tooth-form, each of whose tooth is
approximated by a step form, and having a rotational symmetry with
respect to the optical axis of an incident light, is formed, and
the first phase correction layer and a phase plate which transforms
linearly polarized incident light having a wavelength of
.lamda..sub.1 into circularly polarized light by generating an odd
number times of .pi./2 of birefringent phase difference, are
integrally formed, whereby a transmitted wavefront of light having
a wavelength of .lamda..sub.2, or light having a wavelength of
.lamda..sub.2 and light having a wavelength of .lamda..sub.3
incident in the region of numerical aperture NA.sub.2 are changed
while a transmitted wavefront of light having a wavelength of
.lamda..sub.1 incident in the region of numerical aperture NA.sub.1
is maintained regardless of the polarization state.
2. The phase correction element according to claim 1, further
comprising a first transparent material and a second transparent
material having different refractive index wavelength dispersions
from each other, wherein the difference .DELTA.n of their
refractive indexes is 0 at the wavelength .lamda..sub.1 and finite
values at the wavelength .lamda..sub.2 and the wavelength
.lamda..sub.3, the first transparent material comprises a
saw-tooth-form concavo-convex portion having a cross-sectional
shape of a saw-tooth-form or a saw-tooth-form each of whose tooth
is approximated by a step form, and having a rotational symmetry
with respect to the optical axis of an incident light, at least
concave portions of the concavo-convex portion are filled with the
second transparent material, and the height d of each of convex
portions of the saw-tooth-form satisfies a formula
.lamda..sub.2/2.ltoreq..DELTA.n.times.d.ltoreq..lamda..sub.3
provided that the difference of the refractive indexes at the
wavelength .lamda..sub.2 is .DELTA.n.
3. The phase correction element according to claim 1, wherein each
of convex portions of the saw-tooth-form of the first phase
correction layer is approximated by a step form, and the phase
difference of a transmitted light having a wavelength of
.lamda..sub.1 between a convex portion and a concave portion of
each step of the step form is a natural number times of 4.pi..
4. The phase correction element according to claim 1, wherein the
first phase plate is constructed by laminating two phase plates
having birefringent phase differences of .pi. and .pi./2
respectively at a middle wavelength
.lamda..sub.c=(.lamda..sub.1+.lamda..sub.2)/2 of .lamda..sub.1=410
nm wavelength band and .lamda..sub.2=650 nm wavelength band, so
that the angle between their optical axes is 57.+-.5.degree., and
the first phase plate produces a birefringent phase difference of
an odd number times of .pi./2 at least for linearly polarized
incident light in the .lamda..sub.1 and .lamda..sub.2 wavelength
bands to transform the linearly polarized incident light into
circularly polarized light.
5. The phase correction element according to claim 3, which further
comprises a second phase correction layer in an area of aperture
NA.sub.3 (NA.sub.2>NA.sub.3) in the plane of the phase
correction element, the second phase correction layer comprising a
birefringent material layer having an ordinary refractive index no
and an extraordinary refractive index n.sub.e
(n.sub.e.noteq.n.sub.o) in which the optical axis of a refractive
index ellipsoid is uniformly in one direction, wherein the
birefringent material layer comprises a saw-tooth-form
concavo-convex portion having a saw-tooth-form cross-sectional
shape each of whose convex portions is approximated by a step form,
and having a rotational symmetry with respect to the optical axis
of an incident light, at least concave portions of the
concavo-convex portion are filled with a homogeneous refractive
index transparent material having a refractive index of n.sub.s
approximately equal to the ordinary refractive index n.sub.o or the
extraordinary refractive index n.sub.e, and the phase difference of
extraordinarily polarized transmitted light or ordinarily polarized
transmitted light having a wavelength of .lamda..sub.1
corresponding to the step-height of each step of the step form, is
an odd number times of 2.pi..
6. The phase correction element according to claim 5, wherein the
first phase plate has a function of generating a birefringent phase
difference of an odd number times of .pi./2 for linearly polarized
incident light having a wavelength of .lamda..sub.1 to convert it
to circularly polarized light, and generating a birefringent phase
differences of an odd number times of .pi. for linearly polarized
light having a wavelength of .lamda..sub.2 and linearly polarized
light having a wavelength of .lamda..sub.3 to rotate their
polarization planes.
7. The phase correction element according to claim 6, wherein the
first phase plate has a construction that two phase plates having
birefringent phase differences of .pi./2 and .pi. respectively for
a wavelength .lamda..sub.1, are laminated so that the angle between
their optical axes is 45.+-.5.degree..
8. The phase correction element according to claim 1, wherein the
first phase correction layer comprises a first polarizing phase
correction layer and a second phase correction layer each
comprising a birefringent material layer having an ordinary
refractive index of n.sub.o and an extraordinary refractive index
n.sub.e (n.sub.o.noteq.n.sub.e), in which the optical axis of the
refractive index ellipsoid is uniformly in one direction, the
birefringent material layer has a saw-tooth-form concavo-convex
portion having a cross-sectional shape of a saw-tooth-form or a
shape in which each of convex portions of a saw-tooth-form is
approximated by a step form, and having a rotational symmetry with
respect to the optical axis of an incident light, and at least
concave portions of the concave-concave portion are filled with a
homogeneous refractive index transparent material having a
refractive index of n.sub.s approximately equal to the ordinary
refractive index n.sub.o or the extraordinary refractive index
n.sub.e, wherein the first phase plate has a function of generating
a birefringent phase difference of an odd number times of .pi./2
for linearly polarized light in a .lamda..sub.1=410 nm wavelength
band to convert it to a circularly polarized light, and generating
a birefringent phase difference of an odd number times of .pi. for
linearly polarized light in a .lamda..sub.2=650 nm wavelength band
and linearly polarized light in a .lamda..sub.3=780 nm wavelength
band to rotate their polarization planes, the second phase plate
has a function of generating a birefringent phase difference of an
even number times of .pi. for linearly polarized incident light in
a .lamda..sub.1=410 nm wavelength band without changing the
polarization state, and generating a birefringent phase difference
of an odd number times of .pi. for linearly polarized incident
light in a .lamda..sub.2=650 nm wavelength band and linearly
polarized light in a .lamda..sub.3=780 nm wavelength band to rotate
their polarization planes by 90.degree., and the first phase plate,
the first polarizing phase correction layer, the second phase plate
and the second polarizing phase correction layer are arranged in
this order and integrated.
9. The phase correction element according to claim 1, wherein a
multi-layer film filter which transmits incident light in a
.lamda..sub.1=410 nm wavelength band and incident light in a
.lamda..sub.2=650 nm wavelength band and reflects incident light in
a .lamda..sub.3=780 nm wavelength band, or a diffraction grating
which transmits incident light in a .lamda..sub.1=410 nm wavelength
band and incident light in a .lamda..sub.2=650 nm wavelength band
and diffracts incident light in a .lamda..sub.3=780 nm wavelength
band and has a rectangular cross-sectional shape producing a phase
difference of 10 .pi. for transmitted light in a .lamda..sub.1=410
nm wavelength band between a convex portion and a concave portion,
is formed in an annular region obtained by subtracting a circular
region of a numerical aperture NA.sub.3 from a circular region of a
numerical aperture NA.sub.1 (NA.sub.1>NA.sub.2>NA.sub.3) in
the phase correction element plane.
10. The phase correction element according to claim 1, wherein a
diffraction grating which transmits incident light in a
.lamda..sub.1=410 nm wavelength band and diffracts incident light
in a .lamda..sub.2=650 nm wavelength band and incident light in a
.lamda..sub.3=780 nm wavelength band and has a cross-sectional
shape of concavo-convex form producing a phase difference of 2.pi.
for transmitted light in a .lamda..sub.1=410 nm wavelength band
between a convex portion and a concave portion, is formed in a
first annular region obtained by subtracting a circular region of a
numerical aperture NA.sub.2 from a circular region of numerical
aperture NA.sub.1 in the phase correction element plane, and a
multi-layer film filter which transmits incident light in a
.lamda..sub.1=410 nm wavelength band and incident light in a
.lamda..sub.2=650 nm wavelength band and reflects incident light in
a .lamda..sub.3=780 nm wavelength band, or a diffraction grating
which transmits incident light in a .lamda..sub.1=410 nm wavelength
band and incident light in a .lamda..sub.2=650 nm wavelength band
and reflects incident light in a .lamda..sub.3=780 nm wavelength
band and has a rectangular cross-sectional shape producing a phase
difference of 10.pi. for transmitted light in a .lamda..sub.1=410
nm wavelength band between a convex portion and a concave portion,
is formed in a second annular region obtained by subtracting a
circular region of numerical aperture NA.sub.3 from a circular
region of numerical aperture NA.sub.2 in the phase correction
element plane.
11. A phase correction element wherein the phase difference for
incident light having a wavelength of .lamda..sub.1 is an integer
times of 2.pi. between the annular region as defined in claim 9 in
which the diffraction grating or the multi-layer film filter is
formed or the first and the second annular regions as defined in
claim 10 and the circular region of numerical aperture
NA.sub.3.
12. An optical head device comprising light sources for emitting
light having three wavelengths in a .lamda..sub.1=410 nm wavelength
band, a .lamda..sub.2=650 nm wavelength band and a
.lamda..sub.3=780 nm wavelength band, an objective lens for
converging the light having three wavelengths emitted in an optical
recording medium, and photodetectors for detecting the light
converged and reflected by the optical recording medium, wherein a
phase correction element as defined in claim 1 is disposed in an
optical paths from the light sources for emitting the light having
three wavelengths to the optical recording medium.
Description
TECHNICAL FIELD
[0001] The present invention relates to a phase correction element
and an optical head device, particularly to a phase correction
element to be employed for an optical head device to be used for
recording and/or reproducing an information from three types of
optical recording media employing different wavelengths, and to the
optical head device.
BACKGROUND ART
[0002] Recently, as well known, various types of optical recording
media for recording and/or reproducing information and optical head
devices capable of recording and/or reproducing an information to
the optical recording media have been developed and used.
[0003] Among these, an optical recording medium (hereinafter
referred to as "optical disk") for CD is an optical disk having a
cover thickness of 1.2 mm for protecting the information recording
plane, and a semiconductor laser of a 780 nm wavelength band as a
light source and an objective lens having an NA (numerical
aperture) of from 0.44 to 0.51 are employed for recording and/or
reproducing an information.
[0004] On the other hand, an optical disk for DVD is an optical
disk having a cover thickness of 0.6 mm, and a semiconductor laser
of 650 nm wavelength band as a light source and an objective lens
having a NA of from 0.60 to 0.65 are employed for recording and/or
reproducing an information.
[0005] Further, in order to increase recordable information volume,
an optical disk having a cover thickness of 0.1 mm for which a
semiconductor laser of 410 nm wavelength band as a light source and
an objective lens having a NA of 0.85 are employed, is proposed.
Hereinafter, an optical disk for which a semiconductor laser of 410
nm wavelength band is employed is specifically referred to as an
optical disk for HD.
[0006] Here, light in a .lamda..sub.1=410 nm wavelength band means
light having wavelength from about 390 nm to about 430 nm, light in
a .lamda..sub.2=650 nm wavelength band means light having
wavelength from about 630 nm to about 680 nm, and light in a
.lamda..sub.3=780 nm wavelength band means light having wavelength
from about 760 nm to about 820 nm.
[0007] Further, numerical apertures NA of objective lenses to be
employed for HD, DVD and CD are designated as NA.sub.1, NA.sub.2
and NA.sub.3 respectively. NA.sub.1 is about 0.85, NA.sub.2 is
about from 0.60 to 0.65, and NA.sub.3 is about from 0.44 to
0.51.
[0008] Further, a phase difference caused by the difference between
an ordinary refractive index and an extraordinary refractive index
of a birefringent material for an ordinarily polarized light and an
extraordinarily polarized light respectively, is referred to as
"birefringent phase difference", the terms being used to
distinguish from a normal phase difference corresponding to an
optical path difference not caused by the dependency of refractive
index on polarization. Further, "phase difference" is shown by a
unit of radian (rad), and it is referred to as "wavelength phase
difference" when it is described by a wavelength unit.
[0009] By the way, three types of optical disks for CD, for DVD and
for HD have different cover thicknesses and wavelengths to be used
from one another. Accordingly, there has been a problem that in an
optical head device for recording and/or reproducing an
information, when an objective lens designed for any one type of
optical disk is mounted for recording and/or reproducing an
information from these optical disks compatibly, for example, when
the optical head device is used for recording and/or reproducing an
information from a different type of optical disk from the above
type of optical disk, a large spherical aberration is generated and
recording and/or reproducing of the information can not be
performed.
[0010] To cope with this problem, in order to perform recording
and/or reproducing an information from optical disks having
different cover thicknesses by employing a single objective lens in
the optical head device, various solutions for reducing the
spherical aberration generated have been proposed. (For example,
JP-B-2713257 and JP-B-2725653.)
[0011] As a conventional example, JP-B-2713257 proposes an
aperture-limiting element comprising a substrate and a multi-layer
film filter which is a lamination of transparent dielectric films
having different refractive indexes, or a diffraction grating
formed in the periphery of the substrate. The aperture-limiting
element switches NA by transmitting light having one wavelength and
reflecting or diffracting light having the other wavelength.
[0012] FIG. 20 shows an example of a cross-sectional view of a
conventional aperture-limiting element 1000 which transmits light
having a wavelength of .lamda..sub.2 for DVD and reflects light
having a wavelength of .lamda..sub.3 for CD. A multi-layer film
filter 1200 is formed in an annular region (middle region) obtained
by subtracting a circular region of numerical aperture NA.sub.3
from a circular region of numerical aperture NA.sub.2 on the
surface of a transparent substrate (glass substrate) 1100, which
constitutes an aperture-limiting element transmitting incident
light having a wavelength of .lamda..sub.2 and not transmitting
incident light having a wavelength of .lamda..sub.3.
[0013] Here, a phase compensation film 1300 for phase adjustment is
formed on a multi-layer film filter 1200 having an annular region
so as to align the phases of transmitted light having a wavelength
of .lamda..sub.2 between a circular region of numerical aperture
NA.sub.3 and the annular region in which the multi-layer film
filter 1200 is formed.
[0014] Optical head device reducing spherical aberration caused by
the difference of cover thickness of optical disks, can be
constructed by employing the above aperture-limiting element 1000
together with an objective lens and by switching NA of light beam
to be converged on an information recording plane depending on the
difference of wavelengths for DVD and CD. Here, residual spherical
aberration is reduced by making incident light having a wavelength
of .lamda..sub.3 into the objective lens, to be diverging light
beam.
[0015] As a conventional example 2, JP-B-2725653 proposes a phase
correction element comprising a hologram optical element with an
aperture-limiting function having a concentric circular
interference fringe pattern whose cross-sectional shape is a form
of steps, in addition to the aperture-limiting element. The phase
correction element transmits light having a first wavelength and
diffracts light having a second wavelength different from the first
wavelength to generate a spherical aberration canceling a spherical
aberration of an objective lens.
[0016] Further, an optical head device for recording and/or
reproducing an information in an optical recording medium of an
optical disk such as a CD or a DVD, has a construction that light
emitted from a semiconductor laser as a light source is converged
on the optical recording medium by an objective lens, and returning
light reflected by the optical recording medium is introduced into
a photo-acceptance element as a photodetector by a beam splitter,
and the information in the optical recording medium is transformed
into an electrical signal.
[0017] Here, in order to effectively converge emitting light from
the semiconductor laser on the optical recording medium of the
optical disk and to effectively detect signal light from the
optical recording medium by the photodetector, it is effective to
employ a polarizing beam splitter. The polarizing beam splitter
transmits in an incoming path linearly polarized light emitted from
the light source and having a polarization plane in a predetermined
direction, and reflects or diffracts in a returning path linearly
polarized light reflected by the optical recording medium and
thereby having a polarization plane perpendicular to that of the
incoming path, whereby the polarizing beam splitter can switch the
direction of light to the photodetector. Here, in order to change
the polarization plane of the linearly polarized light in the
returning path perpendicular to the polarization plane in the
incoming path, a phase plate (1/4 waveplate) generating a
birefringent phase difference of .pi./2 for the wavelength of
incident light, is employed, which is disposed in the optical path
between the polarizing beam splitter and the optical recording
medium.
[0018] However, as shown in the conventional examples 1 and 2,
although there is an aperture-limiting element or a phase
correction element applicable for recording and/or reproducing
informations of two types of optical disks by employing a single
objective lens, there is no phase correction element for three
wavelengths applicable for recording and/or reproducing
informations of three types of optical disks of HD, DVD and CD, it
has been difficult to record and/or reproduce informations of these
three types of optical disks by employing a single objective
lens.
[0019] Further, when the aperture-limiting element of the above
conventional example 1 is employed as a compatible element for
three types of optical disks of HD, DVD and CD, it is necessary to
add a function of wavelength selection filter which transmits
incident light having a wavelength of .lamda..sub.1 and does not
transmits incident light having wavelengths of .lamda..sub.2 and
.lamda..sub.3, in a first annular region obtained by subtracting a
circular region of numerical aperture NA.sub.2 for DVD from a
circular region of numerical aperture NA.sub.1 for HD (here,
NA.sub.1>NA.sub.2), of the aperture-limiting element 1000 shown
in FIG. 20. Further, in the same manner, it is necessary to add a
function of wavelength selection filter which transmits incident
light having wavelengths of .lamda..sub.1 and .lamda..sub.2 and
does not transmit incident light having a wavelength of
.lamda..sub.3, in a second annular region obtained by subtracting a
circular region of numerical aperture NA.sub.3 for CD from a
circular region of numerical aperture NA.sub.2 for DVD (here,
NA.sub.2>NA.sub.3). Further, it is necessary for the circular
region of numerical aperture NA.sub.3 to have a function of
transmitting all of incident light having wavelengths of
.lamda..sub.1, .lamda..sub.2 and .lamda..sub.3.
[0020] Then, in a case of applying a conventional technique
employing a multi-layer film filter for the wavelength selection
filter, it is necessary to deposit multi-layer films having
different spectral transmittances in the first annular region and
the second annular region separately in the divided regions, which
requires an extremely complicated process. Therefore, it has been
difficult to stably produce an aperture-limiting element which does
not deteriorate a transmitted wavefront aberration of incident
light having a wavelength of .lamda..sub.1 in the entire region of
aperture NA.sub.1. Here, a transmitted wavefront means a wavefront
of light after the light is transmitted through an optical element
such as a phase correction element. "A transmitted wavefront is
changed" means that a wavefront of light is changed while the light
is being transmitted through an optical element and the light is
output with a changed wavefront.
[0021] Further, a phase correction element producing a birefringent
phase difference of .pi./2 for a wavelength of .lamda..sub.1 and
having non-deteriorated property of phase correction element for
wavelengths of .lamda..sub.2 and .lamda..sub.3 and with which a
phase plate is integrally formed, has been demanded.
[0022] The present invention has been made to solve the
above-mentioned problems, and it is an object of the present
invention to provide a phase correction element and an optical head
device applicable for recording and/or reproducing an information
in three types of optical disks of HD, DVD and CD employing a
single objective lens for HD.
DISCLOSURE OF THE INVENTION
[0023] A first aspect of the present invention provides a phase
correction element for transmitting three kinds of light beams
having three wavelengths .lamda..sub.1, .lamda..sub.2 and
.lamda..sub.3 (.lamda..sub.1<.lamda..sub.2<.lamda..sub.3),
which has an area of numerical aperture NA.sub.2 and an area of
numerical aperture NA.sub.1 including the area of numerical
aperture NA.sub.2 (NA.sub.1>NA.sub.2) in an element plane in
which the light is incident, of the phase correction element;
wherein in the area of numerical aperture NA.sub.2, a first phase
correction layer comprising a concavo-convex portion of a
saw-tooth-form or an approximated saw-tooth-form having a
cross-sectional shape of a saw-tooth-form or a saw-tooth-form, each
of whose tooth is approximated by a step form, and having a
rotational symmetry with respect to the optical axis of an incident
light, is formed, and the first phase correction layer and a phase
plate which transforms linearly polarized incident light having a
wavelength of .lamda..sub.1 into circularly polarized light by
generating an odd number times of .pi./2 of birefringent phase
difference, are integrally formed, whereby a transmitted wavefront
of light having a wavelength of .lamda..sub.2, or light having a
wavelength of .lamda..sub.2 and light having a wavelength of
.lamda..sub.3 incident in the region of numerical aperture NA.sub.2
are changed while a transmitted wavefront of light having a
wavelength of .lamda..sub.1 incident in the region of numerical
aperture NA.sub.1 is maintained regardless of the polarization
state.
[0024] Further, a second aspect of the present invention provides
the phase correction element of the first aspect, wherein the first
phase correction layer comprises a first transparent material and a
second transparent material having different refractive index
wavelength dispersions from each other, wherein the difference
.DELTA.n of their refractive indexes is 0 at the wavelength
.lamda..sub.1 and finite values at the wavelength .lamda..sub.2 and
the wavelength .lamda..sub.3, the first transparent material
comprises a saw-tooth-form concavo-convex portion having a
cross-sectional shape of a saw-tooth-form or a saw-tooth-form each
of whose tooth is approximated by a step form, and having a
rotational symmetry with respect to the optical axis of an incident
light, at least concave portions of the concavo-convex portion are
filled with the second transparent material, and the height d of
each of convex portions of the saw-tooth-form satisfies a formula
.lamda..sub.2/2.ltoreq..DELTA.n.times.d.ltoreq..lamda..sub.3
provided that the difference of the refractive indexes at the
wavelength .lamda..sub.2 is .DELTA.n.
[0025] Further, a third aspect of the present invention provides
the phase correction element of the first aspect, wherein each of
convex portions of the saw-tooth-form of the first phase correction
layer is approximated by a step form, and phase difference of a
transmitted light having a wavelength of .lamda..sub.1 between a
convex portion and a concave portion of each step of the step form
is a natural number times of 4.pi..
[0026] Further, a fourth aspect of the present invention provides
the phase correction element of any one of the first to the third
aspect, wherein the first phase plate is constructed by laminating
two phase plates having birefringent phase differences of .pi. and
.pi./2 respectively at a middle wavelength
.lamda..sub.c=(.lamda..sub.1+.lamda..sub.2)/2 of .lamda..sub.1=410
nm wavelength band and .lamda..sub.2=650 nm wavelength band, so
that the angle between their optical axes is 57.+-.5.degree., and
the first phase plate produces a birefringent phase difference of
an odd number times .pi./2 at least for linearly polarized incident
light in the .lamda..sub.1 and .lamda..sub.2 wavelength bands to
transform the linearly polarized incident light into circularly
polarized light.
[0027] Further, a fifth aspect of the present invention provides
the phase correction element of the third aspect, which further
comprises a second phase correction layer in an area of aperture
NA.sub.3 (NA.sub.2>NA.sub.3) in the plane of the phase
correction element, the second phase correction layer comprises a
birefringent material layer having an ordinary refractive index
n.sub.o and an extraordinary refractive index n.sub.e
(n.sub.e.noteq.n.sub.o) in which the optical axis of a refractive
index ellipsoid is uniformly in one direction, wherein the
birefringent material layer comprises a saw-tooth-form
concavo-convex portion having a saw-tooth-form cross-sectional
shape each of whose convex portions is approximated by a step form,
and having a rotational symmetry with respect to the optical axis
of an incident light, at least concave portions of the
concavo-convex portion are filled with a homogeneous refractive
index transparent material having a refractive index of n.sub.s
approximately equal to the ordinary refractive index n.sub.o or the
extraordinary refractive index n.sub.e, and the phase difference of
extraordinarily polarized transmitted light or ordinarily polarized
transmitted light having a wavelength of .lamda..sub.1
corresponding to the step-height of each step of the step form, is
an odd number times of 2.pi..
[0028] Further, a sixth aspect of the present invention provides
the phase correction element of the fifth aspect, wherein the first
phase plate has a function of generating a birefringent phase
difference of an odd number times of .pi./2 for linearly polarized
incident light having a wavelength of .lamda..sub.1 to convert it
to circularly polarized light, and generating a birefringent phase
differences of an odd number times of .pi. for linearly polarized
light having a wavelength of .lamda..sub.2 and linearly polarized
light having a wavelength of .lamda..sub.3 to rotate their
polarization planes.
[0029] Further, a seventh aspect of the present invention provides
the phase correction element of the sixth aspect, wherein the first
phase plate has a construction that two phase plates having
birefringent phase differences of .pi./2 and .pi. respectively for
a wavelength .lamda..sub.1, are laminated so that the angle between
their optical axes is 45.+-.5.degree..
[0030] Further, an eighth aspect of the present invention provides
the phase correction element of the first aspect, wherein the first
phase correction layer comprises a first polarized phase correction
layer and a second phase correction layer each comprising a
birefringent material layer having ordinary refractive index of
n.sub.o and an extraordinary refractive index n.sub.e
(n.sub.o.noteq.n.sub.e), in which the optical axis of the
refractive index ellipsoid is uniformly in one direction, the
birefringent material layer has a saw-tooth-form concavo-convex
portion having a cross-sectional shape of a saw-tooth-form or a
shape in which each of convex portions of a saw-tooth-form is
approximated by a step form, and having a rotational symmetry with
respect to the optical axis of an incident light, and at least
concave portions of the concave-concave portion are filled with a
homogeneous refractive index transparent material having a
refractive index of n.sub.s approximately equal to the ordinary
refractive index n.sub.o or the extraordinary refractive index
n.sub.e, wherein the first phase plate has a function of generating
a birefringent phase difference of an odd number times of .pi./2
for linearly polarized light in a .lamda..sub.1=410 nm wavelength
band to convert it to a circularly polarized light, and generating
a birefringent phase difference of an odd number times of .pi. for
linearly polarized light in a .lamda..sub.2=650 nm wavelength band
and linearly polarized light in a .lamda..sub.3=780 nm wavelength
band to rotate their polarization planes, the second phase plate
has a function of generating a birefringent phase difference of an
even number times of .pi. for linearly polarized incident light in
a .lamda..sub.1=410 nm wavelength band without changing the
polarization state, and generating a birefringent phase difference
of an odd number times of .pi. for linearly polarized incident
light in a .lamda..sub.2=650 nm wavelength band and linearly
polarized incident light in a .lamda..sub.3=780 nm wavelength band
to rotate their polarization planes by 90.degree., and the first
phase plate, the first polarizing phase correction layer, the
second phase plate and the second polarizing phase correction layer
are arranged in this order and integrated.
[0031] Further, a ninth aspect of the present invention provides
the phase correction element of any one of the first to eighth
aspect, wherein a multi-layer film filter which transmits incident
light in a .lamda..sub.1=410 nm wavelength band and incident light
in a .lamda..sub.2=650 nm wavelength band and reflects incident
light in a .lamda..sub.3=780 nm wavelength band, or a diffraction
grating which transmits incident light in a .lamda..sub.1=410 nm
wavelength band and incident light in a .lamda..sub.2=650 nm
wavelength band and reflects incident light in a .lamda..sub.3=780
nm wavelength band and has a rectangular cross-sectional shape
producing a phase difference of 10 .pi. for transmitted light in a
.lamda..sub.1=410 nm wavelength band between a convex portion and a
concave portion, is formed in an annular shape region obtained by
subtracting a circular region of a numerical aperture NA.sub.3 from
a circular region of a numerical aperture NA.sub.1
(NA.sub.1>NA.sub.2>NA.sub.3) in the phase correction element
plane.
[0032] Further, a tenth aspect of the present invention provides
the phase correction element of any one of the first to eighth
aspect, wherein a diffraction grating which transmits incident
light in a .lamda..sub.1=410 nm wavelength band and diffracts
incident light in a .lamda..sub.2=650 nm wavelength band and
incident light in a .lamda..sub.3=780 nm wavelength band and has a
cross-sectional shape of concavo-convex form producing a phase
difference of 2.pi. for transmitted light in a .lamda..sub.1=410 nm
wavelength band between a convex portion and a concave portion, is
formed in a first annular region obtained by subtracting a circular
region of a numerical aperture NA.sub.2 from a circular region of
numerical aperture NA.sub.1 in the phase correction element plane,
and a multi-layer film filter which transmits incident light in a
.lamda..sub.1=410 nm wavelength band and incident light in a
.lamda..sub.2=650 nm wavelength band and reflects incident light in
a .lamda..sub.3=780 nm wavelength band, or a diffraction grating
which transmits incident light in a .lamda..sub.1=410 nm wavelength
band and incident light in a .lamda..sub.2=650 nm wavelength band
and diffracts incident light in a .lamda..sub.3=780 nm wavelength
band and has a rectangular cross-sectional shape producing a phase
difference of 10.pi. for transmitted light in a .lamda..sub.1=410
nm wavelength band between a convex portion and a concave portion,
is formed in a second annular region obtained by subtracting a
circular region of numerical aperture NA.sub.3 from a circular
region of numerical aperture NA.sub.2 in the phase correction
element plane.
[0033] Further, an eleventh aspect of the present invention
provides the phase correction element of the ninth or tenth aspect,
wherein the phase difference for incident light having a wavelength
of .lamda..sub.1 is an integer times of 2 .pi. between the annular
region of the ninth phase correction element in which the
diffraction grating or the multi-layer film filter is formed or the
first and the second annular regions of the tenth phase correction
element, and the circular region of numerical aperture
NA.sub.3.
[0034] Further, the present invention provides an optical head
device comprising light sources for emitting light having three
wavelengths in a .lamda..sub.1=410 nm wavelength band, a
.lamda..sub.2=650 nm wavelength band and a .lamda..sub.3=780 nm
wavelength band, an objective lens for converging the light having
three wavelengths emitted in an optical recording medium, and
photodetectors for detecting the light converge and reflected by
the optical recording medium, wherein a phase correction element of
any one of the first to eleventh aspect is disposed in an optical
path from the light sources for emitting the light having three
wavelengths to the optical recording medium.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a cross-sectional view showing the construction of
the phase correction element according to the first and the second
embodiments of the present invention.
[0036] FIG. 2 is a plan view showing the construction of the phase
correction element according to the first and the second
embodiments of the present invention.
[0037] FIG. 3 is a graph showing a wavefront aberration of
transmitted light at an optical disk for DVD or for CD, wherein (A)
indicates a wavefront aberration generated by the phase correction
element of the present invention and (B) indicates a wavefront
aberration of transmitted light in an optical disk for DVD or for
CD.
[0038] FIG. 4 is a cross-sectional view showing the construction of
the phase correction element according to the third embodiment of
the present invention.
[0039] FIG. 5 is a partial enlarged view of wavefront aberration
showing a correction function of wavefront aberration by the first
phase correction layer of the phase correction element according to
the third embodiment.
[0040] FIG. 6 is a cross-sectional view showing the construction of
the phase correction element according to the fourth embodiment of
the present invention.
[0041] FIG. 7 is a cross-sectional view showing the construction of
the phase correction element according to the fifth embodiment of
the present invention.
[0042] FIG. 8 is a plan view showing the construction of the phase
correction element according to the fifth embodiment of the present
invention.
[0043] FIG. 9 is a partial enlarged view of wavefront aberration
showing a correction function of wavefront by the second phase
correction layer of the phase correction element according to the
fifth embodiment.
[0044] FIG. 10 is a cross-sectional view showing the construction
of the phase correction element according to the sixth embodiment
of the present invention.
[0045] FIG. 11 is a cross-sectional view showing the construction
of the phase correction element according to the seventh embodiment
of the present invention.
[0046] FIG. 12 is a plan view showing the construction of the phase
correction element according to the seventh embodiment of the
present invention.
[0047] FIG. 13 is an enlarged cross-sectional view showing the
positional relation among processed surfaces of the
aperture-limiting substrate of the seventh phase correction element
shown in FIGS. 11 and 12.
[0048] FIG. 14 is a cross-sectional view showing the construction
of the phase correction element according to a modified example of
the seventh embodiment of the present invention.
[0049] FIG. 15 is a constructual view showing the optical head
device having the phase correction element according to the eighth
embodiment.
[0050] FIGS. 16(a) to 16(c) are cross-sectional views showing light
beams and wavefronts when three types of light having different
wavelengths are incident in the phase correction element according
to the eighth embodiment, wherein FIG. 16(a) is in a case of light
having a wavelength of .lamda..sub.1, 16(b) is in a case of light
having a wavelength of .lamda..sub.2, and 16(c) is in a case of
light having a wavelength of .lamda..sub.3.
[0051] FIGS. 17(a) to 17(c) are cross-sectional views showing light
beam and wavefronts when three types of light having different
wavelengths are incident in the phase correction element according
to the eighth embodiment, wherein FIG. 17(a) is in a case of light
having a wavelength of .lamda..sub.1, 17(b) is in a case of light
having a wavelength of .lamda..sub.2, and 17(c) is in a case of
light having a wavelength of .lamda..sub.3.
[0052] FIGS. 18(a) to 18(c) are cross-sectional views showing light
beams and wavefronts when three types of light having different
wavelengths are incident in the phase correction element according
to the eighth embodiment, wherein FIG. 18(a) is in a case of light
having a wavelength of .lamda..sub.1, 18(b) is in a case of light
having a wavelength of .lamda..sub.2, and 18(c) is in a case of
light having a wavelength of .lamda..sub.3.
[0053] FIG. 19 is a graph showing spectral transmittances of every
region of the aperture-limiting substrate shown in FIG. 12, wherein
(a) shows the spectral transmittance of the first annular region
(A.sub.1), (b) shows the spectral transmittance of the second
annular region (A.sub.2) and (c) shows the spectral transmittance
of the circular region (A.sub.3).
[0054] FIG. 20 is a cross-sectional view showing the construction
of a conventional aperture-limiting element.
BEST MODE FOR CARRYING OUT THE INVENTION
[0055] Now, embodiments of the present invention will be described
with reference to the drawings attached.
First Embodiment
[0056] FIG. 1 is a cross-sectional view and FIG. 2 is a plan view
showing an example of the construction of the first phase
correction element according to the first embodiment of the present
invention.
[0057] A first phase correction element 100 according to this
embodiment is constructed by a first phase correction layer 10A and
a first phase plate 30A each formed in a region of aperture
NA.sub.2.
[0058] The first phase correction layer 10A is formed in an area of
aperture NA.sub.2 corresponding to an optical disk for DVD in the
effective diameter area of aperture NA.sub.1 corresponding to an
optical disk for HD in the phase correction element. The first
phase correction layer 10A comprises a transparent material (first
transparent material) 1A having a refractive index of n.sub.A
formed to have a saw-tooth-like concavo-convex portion having a
cross-sectional shape of a saw-tooth-form (so-called blazed
diffraction grating type) or a saw-tooth-form each of whose convex
portions is approximated by a step-like grating, and having a
rotational symmetry with respect to the optical axis of incident
light, and a transparent material (second transparent material) 1B
of a refractive index of n.sub.B filling at least concave portions
of the concavo-convex portion.
[0059] FIG. 1 shows an example of the construction that the first
phase correction layer 10A is sandwiched between transparent
substrates 5 and 6. However, the construction may be such that the
first phase correction layer 10A is formed on the surface of the
transparent substrate 5 and the concave portions of the first phase
correction layer 10A are not filled with any material. The first
phase correction layer 10A is a wavelength-selection type phase
correction layer which does not change the transmitted wavefront of
incident light having a wavelength of .lamda..sub.1 into an area of
aperture NA.sub.1 in the phase correction layer plane, but changes
the transmitted wavefront of light having a wavelength of
.lamda..sub.2 or a wavelength of .lamda..sub.2 and a wavelength of
.lamda..sub.3, whereby the transmitted wavefront can be changed
depending on the spatial shape distribution of the blazed
diffraction grating.
[0060] The wavelength selection function of the phase correction
layer is performed by using the difference between the transparent
material 1A and the transparent material 1B in the dependency of
refractive index on wavelength or in the dependency of refractive
index on polarization, or by using the dependency of phase
difference on wavelength on the step height of the step-like
grating.
[0061] Further, in the first phase correction element 100, a first
phase plate 30A in which the birefringent phase difference for
light having a wavelength of .lamda..sub.1 is an odd number times
of .pi./2, is sandwiched between the transparent substrate 5 and
the transparent substrate 6 to be integrally formed. The material
of the first phase plate 30A may be any material so long as it has
birefringency. For example, it may be an optical crystal such as a
high-molecular liquid crystal or quartz, or polycarbonate which
exhibits birefringency by a single-axis drawing. A high-molecular
liquid crystal is preferred for the phase plate since it has
relatively large birefringency and can be made uniformly and with
large area as a thin film of at most 50 .mu.m thick sandwiched
between transparent substrates such as glass having a good
flatness.
[0062] A linearly polarized light having a wavelength of
.lamda..sub.1 having a polarization plane at an angle of 45.degree.
to the optical axis of the first phase plate 30A is incident and
shuttles through the first phase plate 30A to be transformed into a
linearly polarized light whose polarization plane is
perpendicular.
[0063] The first phase plate 30A may have a construction of a
single layer of a birefringent material, a construction in which at
least two layers are laminated or a construction that a
high-polymer liquid crystal film is sandwiched between at least two
transparent substrates. The dependency of birefringent phase
difference on wavelength can be controlled by the birefringent
material, or by controlling the dependency of birefringent phase
difference on wavelength by laminating a phase plate.
[0064] For example, the first phase plate 30A may be made to be a
phase plate functioning as a 1/4 waveplate for incident light
having three wavelengths of .lamda..sub.1, .lamda..sub.2 and
.lamda..sub.3, or a phase plate functioning as a 1/4 waveplate for
incident light having a wavelength of .lamda..sub.1 and functioning
as a 1/2 waveplate for incident light having wavelengths of
.lamda..sub.2 and .lamda..sub.3.
[0065] Therefore, by employing the phase correction element of this
embodiment, a wavefront aberration generated when an objective lens
for HD is employed for a DVD or a CD, can be corrected. Further, by
employing it as a phase correction element in an optical head
device having a polarizing beam splitter (PBS) which transmits
linearly polarized incident light having a wavelength of
.lamda..sub.1 and reflects or diffracts linearly polarized incident
light having a polarization plane perpendicular to that of the
above linearly polarized incident light, an optional system having
high light-utilization efficiency can be realized and an optical
head device for recording and/or reproducing with high reliability
can be realized since the emission of a semiconductor laser light
source is stabilized.
Second Embodiment
[0066] Then, a second phase correction element 200 shown in FIG. 1,
comprising a first phase correction layer 10B constituted by
transparent materials having different refractive index wavelength
dispersions, namely a first transparent material 1A and a second
transparent material 1B, and having a refractive index wavelength
dispersion that the refractive index difference .DELTA.n is
substantially zero at a wavelength of .lamda..sub.1 and a definite
value at wavelengths of .lamda..sub.2 and .lamda..sub.3, and the
first phase plate 30B, will be described as follows. Here, the
first phase plate 30B has the same construction as the first phase
plate 30A of the first phase correction element 100.
[0067] The transparent material 1A and the transparent material 1B
are two types of materials having considerably different refractive
index wavelength dispersion in a visible wavelength region, which
have the same refractive index at a wavelength of .lamda..sub.1 and
is transparent at wavelengths of .lamda..sub.1, .lamda..sub.2 and
.lamda..sub.3 and may absorb light at other wavelengths. The
transparent material 1A and the transparent material 1B may be an
inorganic material such as glass, or an organic material employed
as a plastic lens or an optical resin. It may be a composite
material of an inorganic material or an organic material in which
fine particles are dispersed to adjust the refractive index
wavelength dispersion.
[0068] FIG. 3 is a graph showing the wavefront aberration of
transmitted light at an optical disk for DVD or CD.
[0069] In FIG. 3, (A) indicates a wavefront aberration generated
when the second phase correction element 200 of this embodiment is
employed, and (B) indicates a wavefront aberration generated when
the second phase correction element 200 is not employed.
[0070] (B) of FIG. 3 is a graph showing an example of a wavefront
aberration generated when an objective lens for HD having a
numerical aperture of NA.sub.1 designed to have a preferred
aberration for an optical disk for HD having a cover thickness of
0.1 mm in a wavelength region of .lamda..sub.1=410 nm, is employed
with an aperture of NA.sub.2 for an optical disk for DVD having a
cover thickness of 0.6 mm in a .lamda..sub.2=650 nm wavelength
region. This graph shows a wavefront aberration which is a
spherical aberration with a power (magnification) component added.
The horizontal axis shows a numerical aperture NA corresponding to
the aperture diameter and the vertical axis shows the cross section
of a wavelength phase difference which is an optical path
difference of light beam at different NA values with respect to a
ray on the optical axis (NA=0) by a unit of wavelength used. The
wavefront aberration actually has a substantially three-dimensional
shape symmetrical to an axis and has a substantially parabolic
distribution.
[0071] In FIG. 3, a plurality of dotted lines in horizontal
direction show equal-phase wavefronts of an integer times of a
wavelength .lamda..sub.2, and each of the intervals of the
horizontal lines is the wavelength .lamda..sub.2. They are each a
wavelength phase difference obtained by subtracting an integer
times of wavelength .lamda..sub.2 from the wavefront aberration
shown as (B) in FIG. 3, which is a wavefront aberration to be
compensated by a wavefront aberration of at most .lamda..sub.2. In
FIG. 3, (A) shows a wavefront aberration generated when the first
phase correction layer 10B of the second phase correction element
200 of the present invention is employed for compensating the
wavefront aberration of at most .lamda..sub.2, which has a
concentric circular form in which the width of the bottom surface
of a saw-tooth-form narrows from the center towards the
periphery.
[0072] Further, the transparent material 1A processed to have a
cross-sectional shape of a saw-tooth-form concavo-convex portion in
the first phase correction layer 10B, has a Fresnel lens form
having a saw-tooth-like cross-section shown in FIG. 1 and FIG. 2.
The shape is determined as follows.
[0073] Namely, the orbicular zone radius of each convex portion of
the transparent material 1A is determined from a plurality of
orbicular zones obtained by slicing the wavefront aberration shown
in FIG. 3(B) whose cross-sectional shape is substantially parabolic
form and having a substantially paraboloid three-dimensional form,
with an interval of wavelength .lamda..sub.2 in a circular region
of numerical aperture NA.sub.2.
[0074] If these orbicular zones are arranged in a plane of zero
wavefront aberration (a plane perpendicular to the paper in FIG. 3)
in a concentric circular form around an axis of NA=0, the height of
all of these orbicular zones become .lamda..sub.2. Namely, the
transparent material 1A is fabricated so that the optical path
difference between the concave portion and the convex portion in
the interface of the transparent material 1A and the transparent
material 1B at a wavelength of .lamda..sub.2, becomes
.lamda..sub.2.
[0075] When the refractive indexes satisfy n.sub.A>n.sub.B at a
wavelength .lamda..sub.2, it is satisfactory that the transparent
material 1A is processed to have a cross-sectional shape of
saw-tooth-form corresponding to 1A of FIG. 1.
[0076] Further, when the refractive index n.sub.A<n.sub.B at a
wavelength of .lamda..sub.2, the transparent material 1A is
processed to have a cross-sectional shape corresponding to (A) of
FIG. 3 which has a symmetry with respect to a plane perpendicular
to the paper.
[0077] The height d of each of convex portions of the
saw-tooth-form made of the transparent material 1A having a
orbicular zone form corresponding to the wavefront aberration
.lamda..sub.2, is represented by d=.lamda..sub.2/.DELTA.n using a
refractive index difference .DELTA.n between the transparent
material 1A and the transparent material 1B at the wavelength
.lamda..sub.2. When each convex portion of the saw-tooth-form is
approximated by a step form, it is satisfactory that the height d
is in a range satisfying a formula:
.lamda..sub.2/2.ltoreq..DELTA.n.times.d.ltoreq..lamda..sub.2
[0078] Further, in order to correct the aberration for a wavelength
.lamda..sub.2 and a wavelength of .lamda..sub.3, it is satisfactory
that the d is in a range represented by a formula:
.lamda..sub.2/2.ltoreq..DELTA.n.times.d.ltoreq..lamda..sub.3
[0079] Further, the height d is more preferably satisfy a formula:
.lamda..sub.2.ltoreq..DELTA.n.times.d.ltoreq..lamda..sub.3
[0080] Here, when a light having a wavelength .lamda..sub.1 is
incident into the second phase correction element 200, since the
refractive index difference .DELTA.n between the transparent
material 1A and the transparent material 1B at the wavelength
.lamda..sub.1 is zero, the transmitted wavefront does not change.
On the other hand, since the refractive index difference .DELTA.n
with respect to incident light having a wavelength of .lamda..sub.2
is definite, a phase difference .DELTA.n.times.d/.lamda..sub.2 by a
wavelength unit corresponding to the height d of the each convex
portion of the saw-tooth-form, is generated, whereby a change of
transmitted wavefront is formed as shown in (A) of FIG. 3,
correcting the wavefront aberration shown in (B) of FIG. 3.
Further, since the refractive index difference .DELTA.n is definite
also for incident light having a wavelength of .lamda..sub.3, a
phase difference n.times.d/.lamda..sub.3 by a wavelength unit
corresponding to the height d of each convex portion of the
saw-tooth-form is generated, and a change of transmitted wavefront
similar to (A) of FIG. 3 is generated. Namely, it becomes a
transmitted wavefront having a power corresponding to a concave
lens.
[0081] Since the phase correction element functions as a concave
lens for the wavelength .lamda..sub.2 and the wavelength
.lamda..sub.3, the distance between an optical disk and an
objective lens can be made larger, and the stability of an optical
head device in recording and/or reproducing is improved.
[0082] Here, a wavefront aberration generated when the same
objective lens for HD is employed in a .lamda..sub.3=780 nm
wavelength band with an aperture of NA.sub.3 for an optical disk
for CD having a cover thickness of 1.2 mm, is not completely
corrected only by employing the second phase correction element
200. However, by making the incident light having a wavelength of
.lamda..sub.3 slightly divergent with respect to the phase
correction element 200 and the object lens compared with the light
having a wavelength of .lamda..sub.2, good correction of wavefront
aberration can be performed.
[0083] Further, the first phase correction layer 10B may be
processed to have a shape correcting wavefront aberrations of a
wavelength .lamda..sub.2 and wavelength .lamda..sub.3 generated
when both of incident light having a wavelength of .lamda..sub.2
and incident light having a wavelength of .lamda..sub.3 slightly
divergent light beam. In an optical head device comprising a
dual-wavelength laser including semiconductor lasers emitting light
having a wavelength of .lamda..sub.2 and light having a wavelength
of .lamda..sub.3 integrated in a single package as a light source,
and a common collimator lens for making the light of two
wavelengths incident into an objective lens as the same level of
divergent light beam, it is effective to employ a phase correction
element 200 having such a property.
[0084] Therefore, by employing the second phase correction element
200, since the aberration can be corrected only by the difference
of the wavelengths from the wavelength .lamda..sub.1, regardless of
the polarization conditions of the incident light having a
wavelength of .lamda..sub.2 and incident light having a wavelength
of .lamda..sub.3, there is little limitation of the first phase
plate 30B to the wavelength .lamda..sub.2 and the wavelength
.lamda..sub.3.
Third Embodiment
[0085] Then, FIG. 4 is a cross-sectional view showing the
construction of a third phase correction element 300 according to
another embodiment of the present invention.
[0086] Here, the plan view is the same as FIG. 2.
[0087] The phase correction element 300 according to this
embodiment, comprises a first phase correction layer 10C formed in
a region of numerical aperture NA.sub.2 on the surface of a
transparent substrate 5 such as a glass, and a first phase plate
30C formed on one side of a transparent substrate 6 such as a
glass. The first phase plate 30C has the same construction as the
first phase plate 30A of the first phase correction element
100.
[0088] Here, in the same manner as the first phase correction layer
10B of the above second embodiment, the first phase correction
layer 10C generates a wavefront aberration corresponding to (A) of
FIG. 3 for incident light having a wavelength of .lamda..sub.2, and
corrects a wavefront aberration shown in (B) of FIG. 3 generated
when the third phase correction element 300 is not employed. The
first phase correction layer 10C is formed to have a saw-tooth-like
cross-sectional shape (so-called blazed diffraction grating form)
in a region of numerical aperture NA.sub.2 on the surface of the
transparent substrate such as glass, and is constituted by a
multi-step-like blazed diffraction grating made of a homogeneous
material whose convex portions (concavo-convex portion) of a
saw-tooth-form are each approximated by a step-like grating. Here,
the concavo-convex portion is formed to have a shape having a
rotational symmetry with respect to the optical axis.
[0089] Here, the phase difference between transmitted light through
the homogeneous material having a refractive index of n and
transmitted light through the air in each step of the step-like
grating, is made to be substantially a natural number times of
4.pi. for a wavelength .lamda..sub.1. In cases of .lamda..sub.1=410
nm wavelength band and .lamda..sub.3=780 nm wavelength band,
considering the refractive index wavelength dispersion of the
homogeneous material, the wavelength difference becomes
substantially a natural number times of 2 .pi. at the wavelength
.lamda..sub.3 provided that it is substantially a natural number
times of 4.pi. at the wavelength .lamda..sub.1. Therefore, by
approximating each of convex portions of the saw-tooth-form by such
a step-like grating, a first phase correction layer 10C is formed
in which light having a wavelength of .lamda..sub.1 or a wavelength
of .lamda..sub.3 is transmitted without changing the transmitted
wavefront regardless of the polarization conditions of the incident
light, and the transmitted wavefront of incident light having a
wavelength of .lamda..sub.2 is changed.
[0090] Further, the shape of the saw-tooth-like concavo-convex
portion in the first phase correction layer 10C, is a Fresnel lens
form constituted by a step-like grating shown in FIG. 2 and FIG. 4,
and the shape is determined in the same manner as the first phase
correction layer 10B.
[0091] In FIG. 4, the height d.sub.N of the step-like grating of
(N+1) levels (namely N steps) is determined so that an optical path
difference (n-1).times.d.sub.N1 corresponding to the height
d.sub.N1 of one step obtained by equally dividing the height
d.sub.N by N, becomes a natural number times of a .lamda..sub.3=780
nm wavelength band for an optical disk for CD. For example, a case
where (n-1).times.d.sub.N1=.lamda..sub.3 and light having a
wavelength of .lamda..sub.2 is incident into the first phase
correction layer 10C processed to have such a step form, is
considered. In this case, the phase of a transmitted wavefront
becomes: 2 .times. .pi. .times. ( n - 1 ) .times. d N1 / .lamda. 2
= 2 .times. .pi. .times. ( .lamda. 3 / .lamda. 2 ) = 2 .times. .pi.
.times. 1.22 ##EQU1## whereby the transmitted wavefront delays
effectively by 0.22 wavelength per each step of the step-like
grating. Therefore, by approximating the saw-tooth-like
cross-sectional shape by a step form having a steps of N=3 to 5, a
first phase correction layer 10C which only corrects a transmitted
wavefront for an optical disk for DVD is obtained.
[0092] FIG. 5 is a partial enlarged view of wavefront aberration
showing the wavefront aberration correction function of the first
phase correction layer 10C. In FIG. 5, by dividing a wavefront
aberration corresponding to one wavelength .lamda..sub.2 by a
correction amount of optical path difference "a" corresponding to
the height d.sub.N1 of one step of the step-like grating, namely,
a={(n-1).times.d.sub.N1}-.lamda..sub.2 as a unit, the wavefront
aberration is approximately compensated. Here, FIG. 5 shows an
example of correcting the aberration by a five-level (four steps)
step-like grating. The first phase correction layer 10C may be
formed by directly micro-fabricating the surface of the transparent
substrate 5 into a step-like grating, or by fabricating a deposited
film having a desired film thickness.
[0093] Therefore, the third phase correction element 300 has such
an advantage that there is little limitation for materials employed
for the first phase correction layer 10C or that the amount of
fabrication is relatively small since there is a larger difference
of the refractive index than air.
Fourth Embodiment
[0094] Then, FIG. 6 is a cross-sectional view showing an example of
the construction of a fourth phase correction element 400 according
to a fourth embodiment of the present invention. Here, the plan
view is the same as FIG. 2.
[0095] The phase correction element 400 of this embodiment employs
as the first phase plate, a first phase plate 30D comprising two
types of phase plates 3A and 3B made of birefringent materials
having different retardation values, laminated so that their
optical axes are at a predetermined angle to each other. Here, FIG.
6 shows a case where as the first phase correction layer 10D, the
same phase correction layer as the first phase correction layer 10B
of the second phase correction element 200 in FIG. 1 showing the
second embodiment, is employed. However, the first phase correction
layer 10C employed in the third phase correction element 300 of the
third embodiment may also be employed as the first phase correction
layer 10D.
[0096] The phase plates 3A and 3B constituting the first phase
plate 30D, are made of a material having birefringency such as an
optical crystal such as a high-molecular liquid crystal or quartz,
or polycarbonate exhibiting birefringency by single-axis drawing.
The phase plates 3A and 3B may be formed by laminating
high-molecular liquid crystal films having different optical axes
and retardation values on a transparent substrate 6, or by bonding
a phase plate 3A made of polycarbonate to a phase plate 3B made of
a high-molecular liquid crystal film formed on a transparent
substrate 6 employing an adhesive. Further, it may be formed by
employing a phase plate 3B made of quartz instead of the
transparent substrate 6, and forming the phase plate 3A made of a
high-molecular liquid crystal film.
[0097] When the phase plate 3A and the phase plate 3B are disposed
from the side of the phase correction layer 10D in this order, and
the angles of the fast axes of the phase plates 3A and 3B with
respect to a polarization plane of linearly polarized incident
light having a wavelength of .lamda. are designated as
.theta..sub.A and .theta..sub.B respectively, and their retardation
values are designated as R.sub.A and R.sub.B respectively, then the
Stokes matrix component S.sub.3 showing the polarization state of
transmitted light through the laminated phase plate is represented
by the following formula (1).
S.sub.3=Sin(.delta..sub.A).times.[sin(2.theta..sub.A)-{1-cos
(.delta..sub.B)}.times.sin(2.theta..sub.B).times.cos
{2(.theta..sub.A-.theta..sub.B)}]+cos
(.delta..sub.A).times.sin(.delta..sub.B).times.sin(2.theta..sub.B)
(1)
[0098] Here, .delta..sub.A and .delta..sub.B indicate birefringent
phase difference of the phase plates 3A and 3B respectively at a
wavelength of .lamda., and represented by the following formulae:
.delta..sub.A=2.pi.R.sub.A/.lamda.
.delta..sub.B=2.pi.R.sub.B/.lamda.
[0099] Further, the ellipticity .kappa. (the ratio of the minor
axis vibration amplitude based on the major axis vibration
amplitude of an elliptically polarized light) showing the linearity
of polarization of the transmitted light is represented by the
following formula employing S.sub.3:
K=tan{0.5.times.sin-1(S.sub.3)}
[0100] Therefore, in order to form the first phase plate 30D to be
a phase plate having a birefringent phase difference of about an
odd number times of .pi./2 so as to function as a 1/4 waveplate for
incident light having three wavelengths of .lamda..sub.1,
.lamda..sub.2 and .lamda..sub.3, it is satisfactory that
.theta..sub.A, .theta..sub.B, R.sub.A and R.sub.B are determined so
that .kappa. takes 1, namely, S.sub.3 takes a value close to 1 at
each of these three wavelengths.
[0101] For example, a phase plate 3A having a birefringent phase
difference .delta..sub.A.apprxeq..pi., namely, corresponding to 1/2
phase plate for a center wavelength
.lamda..sub.C=(.lamda..sub.1+.lamda..sub.2)/2 of the wavelength
.lamda..sub.1 and the wavelength .lamda..sub.2, and a phase plate
3B having a birefringent phase difference
.delta..sub.B.apprxeq..pi./2, namely corresponding to a 1/4
waveplate, are laminated so that the angle
|.theta..sub.B-.theta..sub.A| between their fast axes is
57.+-.5.degree..
[0102] The angles of the fast axes are
.theta..sub.A=17.+-.5.degree. and .theta..sub.B=74.+-.5.degree., or
.theta..sub.A=74.+-.5.degree. and
.theta..sub.B=17.+-.5.degree..
[0103] By forming the first phase plate 30D to be such laminated
phase plate, it becomes a 1/4 waveplate for three wavelengths
generating a birefringent phase difference of about .pi./2 for
linearly polarized light in a .lamda..sub.1=410 nm wavelength band,
that in a .lamda..sub.2=650 nm wavelength band and that in a
.lamda..sub.3=780 nm wavelength band to convert each of them to
circularly polarized light.
[0104] Here, the above construction of laminating the phase plate
3A and the phase plate 3B constituting the first phase plate 30D,
is an example and the above .theta..sub.A, .theta..sub.B, R.sub.A
and R.sub.B are not limited to the above ranges. .theta..sub.A,
.theta..sub.B, R.sub.A and R.sub.B may be adjusted so as to obtain
a desirable S.sub.3 presented by the formula (1) depending on the
purpose of polarization conversion at each wavelength, since the
dependency of birefringent amount on wavelength differs depending
on birefringent materials employed.
[0105] By employing the fourth phase correction element 400
integrated with such a first phase plate 30D, the fourth phase
correction element 400 functions as a 1/4 waveplate not only for
incident light having a wavelength .lamda..sub.1 but also for
incident light having a wavelength of .lamda..sub.2 and incident
light having a wavelength of .lamda..sub.3.
[0106] For this reason, in a case where the fourth phase correction
element 400 is included in an optical head device to be used for
recording and/or reproducing an information in three types of
optical recording media using different wavelengths, recording
and/or reproducing with an optical recording medium with high
utilization efficiency of light is possible by using a combination
with a polarizing beam splitter. Further, even when a polarizing
beam splitter is not used, since reflected light from the optical
recording medium is transformed into linearly polarized light
having a polarization plane perpendicular to that of emitted light
from a semiconductor laser light source as it shuttles through the
first phase plate 30D, and incident into an emission point of the
laser, it does not affect the emission operation of the
semiconductor laser and stable emission light intensity can be
obtained, which realizes stable recording and/or reproducing with
high reliability.
Fifth Embodiment
[0107] Then, with respect to an example of the construction of a
fifth phase correction element 500 according to the fifth
embodiment of the present invention, FIG. 7 shows a cross-sectional
view and FIG. 8 shows a plan view. Here, FIG. 8(a) shows an
external appearance from the side where the first phase correction
layer 10E is formed, and FIG. 8(b) shows an external appearance
from the other side.
[0108] In the phase correction element 500 according to this
embodiment, a first phase correction layer 10E is formed in a
region of numerical aperture NA.sub.2 on the surface of a
transparent substrate 5 in the same manner as the third phase
correction element 300, a second phase correction layer 20E is
formed in a region of numerical aperture NA.sub.3 on the other
surface of the transparent substrate 5, and the first phase plate
30E is integrally formed. Here, the first phase correction layer
10E has the same construction as the first phase correction layer
10C of the third embodiment.
[0109] First of all, the second phase correction layer 20E will be
described in detail as follows.
[0110] A wavefront aberration obtained by adding a power component
to a spherical aberration generated when an objective lens for HD
having a numerical aperture of NA.sub.1 is employed for an optical
disk for CD having a cover thickness of 1.2 mm in a
.lamda..sub.3=780 nm wavelength band with a numerical aperture of
NA.sub.3, corresponds to (B) of FIG. 3. The method for correcting
the wavefront aberration by using the second phase correction layer
20E, is the same as the above-mentioned procedure for the first
phase correction layers 10A and 10C.
[0111] The second phase correction layer 20E comprises a
transparent substrate 5 such as glass and a high-molecular liquid
crystal layer as a birefringent material layer having an ordinary
refractive index n.sub.o and an extraordinary refractive index
n.sub.e (n.sub.e>n.sub.o) formed in a region of numerical
aperture NA.sub.3 on the transparent substrate 5. Here, a solution
of liquid crystal monomer is applied on an alignment film which is
present on a transparent substrate and subjected to an alignment
treatment, liquid crystal molecules are aligned so that their
alignment vectors (molecular alignment axes) are in parallel in a
specific direction in a plane in parallel to the substrate, and
irradiated with light such as ultraviolet light beam to be
polymerized and cured to form a high-molecular liquid crystal
layer.
[0112] The high-molecular liquid crystal layer is fabricated to be
a concavo-convex portion having a saw-tooth-like cross-sectional
shape (so-called blazed diffraction grating type) in which each of
convex portions of the saw-tooth-form is a blazed diffraction
grating 2A approximated by a step-like grating, and having a
rotational symmetry with respect to the optical axis. Namely, the
concavo-convex portion is formed to have a concentric circular form
in which the width of the bottom surface of the sow-tooth narrows
from center towards the periphery. Then, at least concave portions
of the concavo-convex portion of the high-molecular liquid crystal
layer formed, are filled with a homogeneous refractive index
transparent material 2B having a refractive index n.sub.s
approximately equal to the ordinary refractive index n.sub.o, to
form a second phase correction layer 20E. Namely, a filler of the
homogeneous refractive index transparent material 2B fills a space
between the transparent substrate 5 on which a step-like blazed
diffraction grating 2A constituted by a concavo-convex portion of
high-molecular liquid crystal is formed, and the first phase plate
30E.
[0113] The second phase correction layer 20E receives incident
light having a wavelength of .lamda..sub.1 and incident light
having a wavelength of .lamda..sub.2 as ordinarily polarized
incident light, and incident light having a wavelength of
.lamda..sub.3 as extraordinarily polarized incident light. By
constructing it in this way, a second phase correction layer 20E
can be obtained, which does not change transmitted wavefronts of
light having a wavelength of .lamda..sub.1 and light having a
wavelength of .lamda..sub.2, and changes a transmitted wavefront of
light having a wavelength of .lamda..sub.3 to correct a wavefront
aberration generated to the light having a wavelength
.lamda..sub.3.
[0114] Here, the phase difference between light transmitted through
a high-molecular liquid crystal layer having an extraordinary
refractive index of n.sub.e, and light transmitted through a
homogeneous refractive index transparent material 2B having a
refractive index of n.sub.s in each step of the step-like blazed
diffraction grating 2A, is made to be approximately an odd number
times of 2.pi. with respect to a wavelength of .lamda..sub.1. By
approximating each of the convex portions of the saw-tooth-form by
such a step-like grating, the second phase correction layer 20E is
made, which has a wavelength-selection function of also transmitted
extraordinarily polarized light having a wavelength of
.lamda..sub.1 without changing its transmitted wavefront, and
changing the transmitted wavefront of extraordinarily polarized
light having a wavelength of .lamda..sub.3.
[0115] Further, the cross-sectional shape of the saw-tooth-like
concavo-convex portion in the second phase correction layer 20E, is
made to have a Fresnel lens form constituted by the step-like
grating shown in FIG. 7 and FIG. 8(b) when n.sub.e>n.sub.o, and
it is made to have a step-like grating having a reversed
concavo-convex form when n.sub.e<n.sub.o.
[0116] The shape is determined as follows. Namely, an orbicular
zone radius of each of the convex portions of the blazed
diffraction grating 2A is determined from a plurality of orbicular
zones obtained by slicing the wavefront aberration having a
substantially parabolic cross-sectional shape shown in (B) of FIG.
3 and having a substantially paraboloid three-dimensional shape, at
an interval of a wavelength .lamda..sub.3 in the circular region of
numerical aperture NA.sub.3. By disposing these orbicular zones on
a plane (a plane perpendicular to the paper in FIG. 3) of zero
spherical aberration, in a concentric circular form around an axis
of NA=0, the height of all of these orbicular zones are
.lamda..sub.3 and the cross-sectional shape becomes a
saw-tooth-form.
[0117] Then, in FIG. 7, the optical path difference
(n.sub.e-n.sub.s).times.d.sub.M1 of the height dM.sub.1 of one step
obtained by equally dividing the height d.sub.M of a step-like
grating of (M+1) levels, (namely M steps), is made to be an odd
number times of a .lamda..sub.1=410 nm wavelength band of an
optical disk for HD.
[0118] For example, a case where
(n.sub.e-n.sub.s).times.d.sub.M1=.lamda..sub.1 and light having a
wavelength of .lamda..sub.3 is incident into the second phase
correction layer 20E fabricated to have such a step form is
considered. In this case, the phase of transmitted wavefront
becomes: 2 .times. .pi. .times. ( n e - n s ) .times. d M1 /
.lamda. 3 = 2 .times. .pi. .times. ( .lamda. 1 / .lamda. 3 ) = 2
.times. .pi. .times. 0.52 ##EQU2## whereby the transmitted
wavefront delays effectively by 0.52 wavelength per each step of
the step-like grating.
[0119] Actually, the amount of delay of the transmitted wavefront
becomes smaller than this value considering the refractive index
wavelength dispersion of the high-molecular liquid crystal layer
and the homogeneous liquid crystal transparent material.
Accordingly, by approximating the saw-tooth-like cross-sectional
shape by a step-like grating of M=1 or M=2, a second phase
correction layer 20E is obtained, which corrects the transmitted
wavefront of extraordinarily polarized light for an optical disk
for CD. Here, by making the incident light having an wavelength
.lamda..sub.2 to be an ordinarily polarized light, the incident
light is transmitted without having a change in the transmitted
wavefront by the second phase correction layer 20E since the
ordinary refractive index n.sub.o of the polymer liquid crystal and
the refractive index n.sub.s of the homogeneous refractive index
transparent material becomes approximately equal.
[0120] FIG. 9 is a partial enlarged view of wavefront aberration,
showing the wavefront aberration correction function of the second
phase correction layer 20E at a wavelength .lamda..sub.3. In FIG.
9, by dividing a wavefront aberration corresponding to one
wavelength .lamda..sub.3 by a correction amount of optical path
difference "b" corresponding to the height d.sub.M1 of one step of
the step-like grating, namely: b=(n.sub.e-n.sub.s).times.d.sub.M1
[0121] the wavefront aberration is approximately corrected. Here,
FIG. 9 shows an example of aberration correction by a step-like
grating of three levels (two steps).
[0122] The above explanation is made assuming a case where the
ordinary refractive index n.sub.o of the high-molecular liquid
crystal layer as a birefringent material layer is equal to the
refractive index n.sub.s of the homogeneous refractive index
transparent material. However, in a case where the extraordinary
refractive index n.sub.e is equal to n.sub.s, the same function can
be obtained by considering that the extraordinarily polarized light
and the ordinarily polarized light are exchanged and fabricating
the step-like grating of the high-molecular liquid crystal layer to
have a shape correcting the wavefront aberration generated.
[0123] Here, in the above case, an example of employing a
high-molecular liquid crystal as a birefringent material layer, is
shown. However, the birefringent material may be any material so
long as it has birefringency. For example, it may be an optical
crystal such as quartz or lithium niobate, or an organic material
such as polycarbonate exhibiting birefringency by single axis
drawing. In the birefringent material has an optical axis of the
refractive index ellipsoid is uniformly in one direction. In a case
of e.g. a high-molecular liquid crystal, a molecular alignment axis
corresponds to this.
[0124] Further, in the fifth phase correction element 500, a first
phase correction layer 10E is also formed, which does not change
the wavefront aberration of transmitted light having a wavelength
of .lamda..sub.1 and a wavelength of .lamda..sub.3, and produces a
wavefront aberration change correcting the wavefront aberration
only for transmitted light having a wavelength of
.lamda..sub.2.
[0125] Therefore, a wavefront aberration generated when an
objective lens for HD is employed for an optical disk for DVD with
a wavelength .lamda..sub.2 and a numerical aperture NA.sub.2, can
be corrected by the first phase correction layer 10E, and a
wavefront aberration generated when it is employed for an optical
disk for CD with a wavelength .lamda..sub.3 and a numerical
aperture NA.sub.3, can be corrected by the second phase correction
layer 20E. These corrections can be made independently from each
other.
[0126] Namely, the second phase correction layer 20E employed for
the fifth phase correction element 500, does not change the
transmitted wavefront of light having a wavelength of .lamda..sub.1
regardless of the polarization state. On the other hand, it does
not change the transmitted wavefront of ordinarily polarized light
having a wavelength of .lamda..sub.2 and a wavelength of
.lamda..sub.3. However, it changes the transmitted wavefront of
extraordinarily polarized light depending on the shape of the
blazed diffraction grating 2A having a step-form.
[0127] Therefore, when the fifth phase correction element 500 is
employed as it is integrated with an objective lens and mounted in
an optical head device, and when polarizations of incident light
having a wavelength of .lamda..sub.2 and incident light having a
wavelength of .lamda..sub.3 in an incoming path of light
propagation from the light source to the optical disk, are made to
be ordinary polarization and extraordinary polarization
respectively, a transmitted wavefront of only the incident light
having a wavelength of .lamda..sub.3 is changed in the second phase
correction layer 20E so that a desired wavefront aberration is
corrected.
[0128] However, since light reflected by the optical disk and
entering into the phase correction element 500 in the returning
path shuttles through the first phase plate 30E, it usually has a
different polarization state from that in the incoming path. When
an extraordinary polarization component of light having a
wavelength of .lamda..sub.2 is generated, the transmitted wavefront
through the second phase correction layer 20E is changed and a
wavefront aberration is generated. Further, when the ordinary
polarization component of wavelength .lamda..sub.3 is generated,
the original wavefront aberration remains since the second phase
correction layer 20E does not change a transmitted wavefront so as
to correct the wavefront aberration. As a result, a problem that a
sufficient signal light can not be collected in a photo-acceptance
plane of a photodetector in the returning path.
[0129] For example, in a case of employing a conventional 1/4
waveplate as the first phase plate which produces a birefringent
phase difference of .pi./2 at a wavelength .lamda..sub.1, there
remains about .pi./2 of birefringent phase difference after the
reciprocation of light having a wavelength .lamda..sub.2 and a
wavelength .lamda..sub.3 through this phase plate, which produces a
polarization component for generating a wavefront aberration.
[0130] Therefore, in order to solve such problem, the first phase
plate 30E is employed, which generates a birefringent phase
difference of about an odd number times of .pi./2 for linearly
polarized light in a .lamda..sub.1=410 nm wavelength band to
transform it into circularly polarized light, and generates a
birefringent phase difference of about an odd number times of .pi.
for linearly polarized light in a .lamda..sub.2=650 nm wavelength
band and linearly polarized light in a .lamda..sub.3=780 nm
wavelength band to rotate their polarization planes. An example of
the construction is described using a cross-sectional view shown in
FIG. 7.
[0131] In a case where the first phase plate 30E has a dual layer
construction comprising two types of phase plates 3C and 3D made of
birefringent materials having different retardation values which
are disposed in this order from the side of the second phase
correction layer 20E wherein the angles of the optical axes of the
phase plates 3C and 3D with respect to the polarization plane of
the linearly polarized incident light having a wavelength of
.lamda., are designated as .theta..sub.C and .theta..sub.D
respectively and their retardation values are designated as R.sub.C
and R.sub.D respectively, it is satisfactory that .theta..sub.C,
.theta..sub.D, R.sub.C and R.sub.D are determined so that the
ellipticity .kappa. calculated from S.sub.3 as the Stokes matrix
component of transmitted light through a laminated waveplate
represented by the above formula (1), becomes substantially 1 at a
wavelength .lamda..sub.1, and at most 0.1 at a wavelength
.lamda..sub.2 and a wavelength .lamda..sub.3.
[0132] Specifically, the phase plate 3C having a birefringent phase
difference of .delta..sub.C.apprxeq..pi./2, namely, corresponding
to a 1/4 phase plate at a wavelength .lamda..sub.1, and a phase
plate 3D having a birefringent phase difference of
.delta..sub.D.apprxeq..pi., namely, corresponding to a 1/2
waveplate, are laminated so that the angle
|.theta..sub.D-.theta..sub.C| formed between their optical axes
becomes 45.+-.5.degree.. The angle of the optical axis of the phase
plate 3C is made to be .theta..sub.C.apprxeq.45.+-.5.degree.. By
constructing such a laminated phase plate, a desired first phase
plate 30E for three wavelengths can be obtained.
[0133] Therefore, ordinarily polarized light having a wavelength of
.lamda..sub.1 becomes extraordinarily polarized light after it
comes and returns through the first phase plate 30E in the fifth
phase correction element 500, ordinarily polarized light having a
wavelength of .lamda..sub.2 remains in ordinarily polarized, and
ordinarily polarized light having a wavelength of .lamda..sub.3
remain in extraordinarily polarized light. As a result, the
transmitted wavefront of a wavelength .lamda..sub.1 does not change
in the incoming path and the returning path, the transmitted
wavefront of a wavelength .lamda..sub.2 is corrected only by the
first phase correction layer 10E, and the transmitted wavefront of
a wavelength .lamda..sub.3 is corrected only by the second phase
correction layer 20E.
Sixth Embodiment
[0134] Then, FIG. 10 shows a cross-sectional view of an example of
the construction of the sixth phase correction element 600
according to the sixth embodiment of the present invention.
[0135] The phase correction element 600 according to this
embodiment comprises a first phase plate 30F generating a
birefringent phase difference of substantially an odd number times
of .pi./2 for linearly polarized light in a .lamda..sub.1=410 nm
wavelength band to transform it into circularly polarized light and
generating a birefringent phase difference of substantially an odd
number times of .pi. for linearly polarized light in a
.lamda..sub.2=650 nm wavelength band and in a .lamda..sub.3=780 nm
wavelength band to rotate their polarization planes, and a second
phase plate 40F generating a birefringent phase difference of
substantially an even number times of .pi. for linearly polarized
light in a .lamda..sub.1=410 nm wavelength band and generating a
birefringent phase difference of substantially odd number times of
.pi. for linearly polarized light in a .lamda..sub.2=650 nm
wavelength band and linearly polarized light in a .lamda..sub.3=780
nm wavelength band to rotate their polarization planes by
substantially 90.degree., a first polarizing phase correction layer
10F.sub.1 and a second polarizing phase correction layer 10F.sub.2,
wherein the first phase plate 30F, the first polarizing phase
correction layer 10F.sub.1, the second phase plate 40F and the
second polarizing phase correction layer 10F.sub.2 are disposed in
this order. Here, numerical references 51 and 52 indicate
transparent substrates on which polarizing phase correction layers
10F.sub.2 and 10F.sub.1 are formed.
[0136] On transparent substrates 52 and 51 such as glass,
high-molecular liquid crystal layers as birefringent materials each
having an ordinary refractive index of n.sub.o and an extraordinary
refractive index of n.sub.e in which the optical axes are aligned
in one direction, are formed in a region of the numerical aperture
NA.sub.2. Each of the high-molecular liquid crystal layers is
constituted by saw-tooth-like gratings 1D or 1F which is processed
to have a saw-tooth-like concavo-convex portion having a
cross-sectional shape of a saw-tooth-form or each convex portion of
the saw-tooth-form being approximated by step-like grating, and
having a rotational symmetry with respect to the optical axis. At
least concave portions of the concavo-convex portion of the
high-molecular liquid crystal layers are filled with homogeneous
refractive index transparent materials 1E and 1G having a
refractive index n.sub.s substantially equal to the ordinary
refractive index n.sub.o.
[0137] With this construction, the first polarizing phase
correction layer 10F.sub.1 is constituted by the saw-tooth-like
grating 1D and the homogeneous refractive index transparent
material 1E, and the second polarizing phase correction layer
10F.sub.2 is constituted by the saw-tooth-like grating 1F and the
homogeneous refractive index transparent material 1G.
[0138] As shown in FIG. 10, slopes of saw-teeth of the
saw-tooth-like grating 1F and vertical faces of the sow-teeth of
the saw-tooth-like grating 1D face the central axis of the
concentric circular grating pattern. Therefore, in a case of
n.sub.e>n.sub.o, when a plane wave of extraordinarily polarized
light is incident, the wavefront of the light transmitted through
the saw-tooth-like grating 1F becomes a divergent spherical wave,
and the wavefront of light transmitted through the saw-tooth-like
grating 1D becomes a convergent spherical wave. The saw-tooth-like
gratings 1F and 1D have functions of a concave lens and a convex
lens respectively.
[0139] Further, in the polarizing phase correction layers 10F.sub.1
and 10F.sub.2, directions of the high-molecular liquid crystal
processed to be the saw-tooth-like gratings 1D and 1F, are aligned,
and the transmitted wavefront does not change for ordinarily
polarized light but the transmitted wavefront is changed for
extraordinarily polarized light depending on a distribution of
saw-tooth-like concavo-convex portion.
[0140] Here, in a case of n.sub.e<n.sub.o, the saw-tooth-like
gratings 1F and 1D be processed so that they have opposite
concavo-convex forms. Further, in a case of n.sub.e=n.sub.s,
exchanging of the extraordinarily polarized and the ordinarily
polarized light is considered, and the same function can be
obtained by processing the saw-tooth-like gratings 1F and 1D to
have concavo-convex forms shown in FIG. 10 in a case of
n.sub.e<n.sub.o and by processing them to have inversed
concavo-convex forms in a case of n.sub.e>n.sub.o.
[0141] Therefore, the wavefront aberration shown in FIG. 3(B)
generated when an objective lens for HD is employed for an optical
disk for DVD with a wavelength of .lamda..sub.2 and a numerical
aperture of NA.sub.2, or when it is used for an optical disk for CD
with a wavelength of .lamda..sub.3 and a numerical aperture
aperture of NA.sub.3, can be corrected by processing the
high-molecular liquid crystal layer 1F so that a wavefront
aberration generated to extraordinarily polarized light having a
wavelength of .lamda..sub.2 and that having a wavelength of
.lamda..sub.3 transmitted through the polarizing phase correction
layer 10F.sub.2, corresponds to (A) of FIG. 3. This is the same as
the case of the correction by the first phase correction layer 10B
in the second phase correction element 200 shown in FIG. 1.
[0142] On the other hand, the first phase plate 30F has the same
construction and function as the first phase plate 30E employed in
the fifth phase correction element 500, and functions as a 1/4
phase plate for linearly polarized light having a wavelength of
.lamda..sub.1 and transform it to circularly polarized light, and
functions as a 1/2 phase plate for linearly polarized light having
a wavelength of .lamda..sub.2 and that having a wavelength of
.lamda..sub.3 to rotate their polarization planes.
[0143] Further, the second phase plate 40F passes the light of
wavelength .lamda..sub.1 maintaining the polarization state of the
incident light, and functions as a 1/2 phase plate rotating the
polarization planes by about 90.degree. for linearly polarized
light having a wavelength of .lamda..sub.2 and that having a
wavelength of .lamda..sub.3.
[0144] In the specific construction of the second phase plate 40F,
the phase plate 3E and the phase plate 3F each having a
birefringent phase difference of about 2.pi. corresponding to about
one wavelength for a wavelength of .lamda..sub.1, are laminated so
that their optical axes are at an angle of about 45.degree. to each
other. Namely, when the birefringent phase differences of the phase
plates 3E and 3F at a wavelength .lamda..sub.1 are .delta..sub.E
and .delta..sub.F respectively and the angles of their fast axes to
the polarization plane of the incident light are .theta..sub.E and
.theta..sub.F respectively, they have the following relations.
.delta..sub.E=.delta..sub.F.apprxeq.2.pi. and
|.theta..sub.F-.theta..sub.E=45.+-.5.degree..
[0145] In an incoming path where incident light from a light source
is converged on an optical disk, linearly polarized light in a
.lamda..sub.1=410 nm wavelength band is incident into the second
polarizing phase correction layer 10F.sub.2 as ordinarily polarized
light, and is straightly transmitted without being diffracted, and
is transmitted through the second phase plate 40F as it is
ordinarily polarized light. Accordingly, it is transmitted through
the first polarizing phase correction layer 10F.sub.1 without being
diffracted and transmitted through the first phase plate 30F to be
circularly polarized light.
[0146] Further, linearly polarized light in a .lamda..sub.2=650 nm
wavelength band and that in a .lamda..sub.3=780 nm wavelength band,
are each incident into the second polarizing phase correction layer
10F.sub.2 as an extraordinarily polarized light, diffracted and
transmitted through the second phase plate 40F to be ordinarily
polarized light, straightly transmitted through the first
polarizing phase correction layer 10F.sub.1 without being
diffracted, and is transmitted through the first phase plate 30F
with the polarization plane rotated.
[0147] Here, the blazed diffraction grating formed on the second
polarizing phase correction layer 10F.sub.2, has preferably a
blazed grating having a saw-tooth-form in cross section and is
adapted so that a phase difference of wavefronts of light
transmitted through the concavo-convex portion of the grating is
about one wavelength of the wavelength .lamda..sub.2 or the
wavelength of .lamda..sub.3 between the concave portion and the
convex portion. Further, the blazed diffraction grating has a
concentric orbicular-zone-like grating pattern formed to correct a
spherical aberration generated by a first order diffraction light
in an optical head device.
[0148] On the other hand, in a returning path where light reflected
by an optical disk is converged on a photodetector, linearly
polarized light having a wavelength of .lamda..sub.1 come and
returns through the first phase plate 30F to be an extraordinarily
polarized light and is incident into the first polarizing phase
correction layer 10F.sub.1. Then the light is diffracted and
transmitted through the second phase plate 40F as it is the
extraordinarily polarized light, and diffracted again in the second
polarizing phase correction layer 10F.sub.2.
[0149] Further, light having a wavelength of .lamda..sub.2 and
light having a wavelength of .lamda..sub.3 are each reformed into
the original ordinarily polarized light by being rotated
polarization plane in the phase plate 30F, and straightly
transmitted through the first polarizing phase correction layer
10F.sub.1 without being diffracted, and transmitted through the
second phase plate 40F to be extraordinarily polarized light, and
diffracted by the second polarizing phase correction layer
10F.sub.2 in the same manner as the incoming path. Here, the second
phase correction layer 10F.sub.2 is formed so as to have a high
first order diffraction efficiency to extraordinarily polarized
lights having a wavelength of .lamda..sub.2 and that having a
wavelength of .lamda..sub.3, and accordingly, second order
diffraction light is mainly generated when ordinarily polarized
light having a wavelength of .lamda..sub.1 is incident.
[0150] Accordingly, the blazed diffraction grating pattern of the
first polarizing phase correction layer 10F, is formed to have the
same wavefront state as the incident light to the phase correction
element 600 in the incoming path by the multi-diffraction of the
diffraction light having a convergent transmitted wavefront
generated at the first polarizing phase correction layer 10F.sub.1
and the second polarizing phase correction layer 10F.sub.2 when the
extraordinarily polarized light having a wavelength .lamda..sub.1
is incident in the returning paths. In this case, the order number
of the diffraction light of a wavelength of .lamda..sub.1 by the
first polarizing phase correction layer 10F.sub.1, may be first or
second. However, its diffraction direction with respect to the
central axis of the concentric circular grating pattern is opposite
from that of the second polarizing phase correction layer
10F.sub.2.
[0151] With the construction of the phase correction element 600 of
this embodiment, transmitted wavefront of ordinarily polarized
incident light having a wavelength of .lamda..sub.1 is not changed
in the incoming and returning paths, and is transformed into an
extraordinarily polarized light that is perpendicular to the
incident light after it come and returns.
[0152] On the other hand, the function of changing the transmitted
wavefront of extraordinarily polarized incident light having a
wavelength of .lamda..sub.2 and that having a wavelength of
.lamda..sub.3 so as to correct the wavefront aberration in the
incoming path and the returning path can be obtained. Namely, by
combining the second polarizing phase correction layer 10F.sub.2,
the second phase plate 40F and the first polarizing phase
correction layer 10F.sub.1, the same function as that of the first
polarizing phase correction layer 10B in the second phase
correction element 200 can be obtained.
[0153] When the above-mentioned first to sixth phase correction
elements 100 to 600 are mounted as compatible elements for three
types of optical disks of HD, DVD and CD in optical head devices,
it is preferred to combine an aperture-limiting element for
limiting incident light beam of wavelengths .lamda..sub.1,
.lamda..sub.2 and .lamda..sub.3 to be numerical apertures NA.sub.1,
NA.sub.2 and NA.sub.3 respectively. The aperture-limiting element
may be disposed separately from the phase correction element of the
present invention. However, it is preferred to form the phase
correction element to have the aperture-limiting function whereby
the device can thereby be small in size and light in weight, and
adjustment work for fitting become unnecessary.
[0154] Further, in the phase correction element of the present
invention, a light beam of a wavelength of .lamda..sub.2 whose
transmitted wavefront is changed by the first phase correction
layer formed in the area corresponding to the numerical aperture
NA.sub.2 for DVD, is converged in a focal plane of an objective
lens different from that of a light beam of a region outside the
area of a numerical aperture NA.sub.2 when a large power component
is applied to the transmitted wavefront, in addition to a spherical
aberration correction component. Namely, when the light beam of the
numerical aperture NA.sub.2 is converged on an information
recording plane of an optical disk, it is not detected as a signal
light by the photodetector of the optical head device since the
light beam of an outer area is not converged on the information
recording plane, and as a result, the first phase correction layer
has an aperture-limiting function of the numerical aperture of
NA.sub.2 to incident light having a wavelength of
.lamda..sub.2.
[0155] In the same manner, in the fifth phase correction element
500 shown in FIG. 7, the light beam of a wavelength .lamda..sub.3
whose transmitted wavefront is changed by the second phase
correction layer 20E formed in the region corresponding to the
numerical aperture NA.sub.3 for CD, is converged on a focal plane
of an objective lens different from that of the light beam of the
outer area of the numerical aperture NA.sub.3 if a large power
component is added to the transmitted wavefront. Namely, it is not
detected as a signal light by the photodetector of the optical head
device, and as a result, the second phase correction layer 20E has
an aperture-limiting function of the numerical aperture NA.sub.3 to
incident light having a wavelength .lamda..sub.3.
[0156] In a case where the first phase correction layer and the
second phase correction layer have functions of generating large
power components, it is not necessary to add an additional
aperture-limiting function to the phase correction element.
[0157] However, in a case where there is little power component
imparted to the transmitted wavefront by the phase correction layer
and a light beam out of a predetermined numerical aperture is
incident into the photodetector as a stray light, stable recording
and/or reproducing can not be performed. In particular, in a case
of a phase correction element having only the first phase
correction layer formed in the region corresponding to an aperture
of NA.sub.2, it is preferred to provide an aperture-limiting
function for limiting the light beam of a wavelength of
.lamda..sub.3 for CD to NA.sub.3.
[0158] Then, various embodiments of the aperture limitation formed
integrally with the phase correction element of the present
invention, will be described.
Seventh Embodiment
[0159] With respect to the seventh phase correction element 700
according to the seventh embodiment of the present invention, a
cross-sectional view is shown in FIG. 11 and a plan view is shown
in FIG. 12.
[0160] On a surface of a transparent substrate 5, a diffraction
grating 91 having a cross-sectional shape of a rectangular
concavo-convex form with a ratio of the lengths between the concave
portion and the convex portion of 1:1, and having a wavelength
phase difference corresponding to a wavelength .lamda..sub.1, is
formed in a first orbicular region (A.sub.1) which is the
difference between a circular region of aperture NA.sub.1 and a
circular region of aperture NA.sub.2. This construction provides an
aperture-limiting function that since the wavelength phase
difference between the concave portion and the convex portion is
substantially 1/2 of the wavelength .lamda..sub.2 and the
wavelength .lamda.3, incident light having a wavelength of
.lamda..sub.1 is straightly transmitted and incident light having a
wavelength of .lamda..sub.2 and incident light having a wavelength
of .lamda..sub.3 are diffracted, whereby straightly transmitted
light becomes at most 30%. The same wavelength-selection function
is exhibited when the wavelength phase difference between the
concave portion and the convex portion is substantially an integer
times of the wavelength .lamda..sub.1 and it is non-integer times
of the wavelength .lamda..sub.2 and the wavelength .lamda..sub.3,
preferably a value close to an odd number times of 1/2 of these
wavelengths.
[0161] Further, in a second annular region (A.sub.2) which is the
difference between the circular region of aperture NA.sub.2 and the
circular region of aperture NA.sub.3 on the surface of the
transparent substrate 5, a multi-layer film filter 92 having such
construction that a transparent dielectric film (not shown) having
a relatively high refractive index and a transparent dielectric
film (not shown) having a relatively low refractive index are
alternately laminated wherein each of the optical films is in the
order of the wavelength, is formed.
[0162] In the multi-layer film filter 4A, the refractive indexes,
the number of layers of two types of transparent dielectric
material and the film thickness of each layer are determined
according to a conventional multi-layer film filter design
procedure so that the filter transmits at most 90% of light having
a wavelength of .lamda..sub.1 and light having a wavelength of
.lamda..sub.2 and reflects at most 70% of light having a wavelength
of .lamda..sub.3. As the transparent dielectric material film
having a high refractive index, TiO.sub.2, Ta.sub.2O.sub.5,
Nb.sub.2O.sub.5, ZrO.sub.2 or the like is employed, and as the
transparent dielectric film of low refractive index, SiO.sub.2,
MgF.sub.2 or the like is employed.
[0163] With the surface of the transparent substrate 5 having such
structure, an aperture-limiting substrate 5A having a
wavelength-selectivity by which incident light having a wavelength
of .lamda..sub.1 is straightly transmitted through the area of
numerical aperture NA.sub.1 and incident light having a wavelength
of .lamda..sub.2 is straightly transmitted in a region of numerical
aperture NA.sub.2, and incident light having a wavelength of
.lamda..sub.3 is straightly transmitted through a region of
numerical aperture NA.sub.3. Here, the seventh phase correction
element 700 may comprise any one of the first to the sixth phase
correction element in the construction except for the
aperture-limiting substrate 5A. FIG. 11 shows a case of employing
the fourth phase correction element 400.
[0164] Here, it is preferred to provide a step for phase correction
in at least one region from the first annular region (A.sub.1), the
second annular region (A.sub.2) and the annular region (A.sub.3) on
the surface of the aperture-limiting substrate 5A so that a
wavefront of light having a wavelength of .lamda..sub.1 transmitted
through the region of numerical aperture NA.sub.1 (including all
inside areas) of the aperture-limiting substrate 5A, does not
change and a wavefront of light having a wavelength of
.lamda..sub.2 transmitted through the region of numerical aperture
NA.sub.2 (including all inside areas) does not change.
[0165] There is a case that the step for phase adjustment is formed
by directly processing the transparent substrate 5, and a case that
it is formed after the transparent dielectric film is formed on the
surface of the transparent substrate 5. In both cases, it is
preferred to form an antireflection film 8 to impart antireflective
function so that the circular region (A.sub.3) transmits three
types of incident light having wavelengths of .lamda..sub.1,
.lamda..sub.2 and .lamda..sub.3, respectively.
[0166] Then, the aperture-limiting substrate 5A of FIG. 11 is
specifically described using FIG. 13 of partially enlarged
cross-sectional view.
[0167] FIG. 13 shows a case where the surface (S.sub.0 surface) of
the annular region (A.sub.2) is processed to have a stepped surface
for phase adjustment so that the step interval with respect to the
surface of the circular region (A.sub.3) is d.sub.3 by processing
the surface of the transparent substrate 5.
[0168] A diffraction grating 91 having a concavo-convex like
cross-sectional shape is formed in the annular region (A.sub.1),
wherein the surface of the concave region is designated as S.sub.1
and the distance between S.sub.0 and S.sub.1 is designated as
d.sub.1. Further, the surface of the convex portion of the
concavo-convex like diffraction grating 91 is designated as
S.sub.4, and the distance between S.sub.0 and S.sub.4 is designated
as d.sub.4. Further, a multi-layer film filter 92 is formed only in
the circular region (A.sub.2), and the surface of the multi-layer
film filter 92 is designated as S.sub.2, and the distance between
the S.sub.0 and S.sub.2, namely the film thickness of the
multi-layer film filter 92, is designated as d.sub.2.
[0169] Here, in order to prevent the transmitted wavefront of an
incident light having a wavelength of .lamda..sub.1 straightly
transmitted through a region of numerical aperture NA.sub.1 from
being changed to generate a wavefront aberration, each of the
circular region (A.sub.1), the circular region (A.sub.2) and the
circular region (A.sub.3) is constructed to produce a wavelength
phase difference of an integer times of the wavelength
.lamda..sub.1 for light having a wavelength of .lamda..sub.1
transmitted through the region.
[0170] Further, in order to prevent the wavefront of an incident
light having a wavelength of .lamda..sub.2 straightly transmitted
through the region of aperture NA.sub.2 from being changed to cause
a wavefront aberration, each of the annular region (A.sub.2) and
the circular region (A.sub.3) is constructed to produce a
wavelength phase difference of an integer times of the wavelength
of .lamda..sub.2 for light having the wavelength of .lamda..sub.2
transmitted through the region.
[0171] A diffraction grating 91 in the annular region (A.sub.1)
being in contact with the air, is processed so that the depth of
the concave portion of the refractive index n becomes
(d.sub.4-d.sub.1)=.lamda..sub.1/(n-1) so that the wavelength phase
difference between a concave portion and a convex portion become
the wavelength .lamda..sub.1, and an incident light having a
wavelength of .lamda..sub.1 is straightly transmitted without being
diffracted, and transmitted wavefronts through the concave portion
and the convex portion of the diffraction grating 91 are in
phase.
[0172] Therefore, the wavelength phase difference of the
transmitted light having a wavelength of .lamda..sub.1 caused by
the difference between the optical path between S.sub.0 surface and
S.sub.1 or S.sub.4 surface of the annular region (A.sub.1) and an
optical path between S.sub.0 surface and S.sub.3 surface of the
circular region (A.sub.3), have to be an integer number times of
wavelength .lamda..sub.1.
[0173] As a result, since no phase difference is generated between
the annular region (A.sub.1) and the circular region (A.sub.3) with
respect to the transmitted light of wavelength .lamda..sub.1, only
the phase difference of transmitted light having a wavelength of
.lamda..sub.1 and light having a wavelength .lamda..sub.2 between
the annular region (A.sub.2) and the circular region (A.sub.3),
have to be adjusted.
[0174] When the multi-layer film filter 92 having a total film
thickness of d.sub.2 in the annular region (A.sub.2) is assumed to
be a homogeneous layer having an average refractive index n.sub.M,
and an increase of the optical path by a multiple reflection at the
interface between films having different refractive indexes inside
of the multi-layer film filter 92 is taken account as an average
refractive index n.sub.M, the optical paths L.sub.2 and L.sub.3
from S.sub.0 surface to S.sub.2 and S.sub.3 surfaces in the annular
region (A.sub.2) and the circular region (A.sub.3), are represented
by the following formulae (2) and (3) respectively:
L.sub.2=n.sub.Md.sub.2+(d.sub.3-d.sub.2) (2) L.sub.3=n.sub.3d.sub.3
(3)
[0175] Here, n.sub.3 indicates the average refractive index of a
portion between S.sub.0 surface and S.sub.3 surface including an
antireflective film 8 in the circular region (A.sub.3).
[0176] Therefore, by processing S.sub.0 surface to form S.sub.3
surface so as to satisfy the relation between the construction of
the multi-layer filter 92 (total film thickness d.sub.2 and average
refractive index n.sub.M) and the construction of the circular
region (A.sub.3) (total film thickness d.sub.3 and average
refractive index n.sub.3) so that the wavelength phase difference
of (L.sub.2-L.sub.3) becomes about an integer number times of the
wavelength .lamda..sub.1 and about an integer times of the
wavelength .lamda..sub.2, the transmitted wavefronts of light
having a wavelength of .lamda..sub.1 and light having a wavelength
of .lamda..sub.2 through the NA.sub.2 region do not change, and no
wavefront aberration by the aperture-limiting substrate 5A is
generated. As a result, when the seventh phase correction element
700 is employed together with an objective lens of an optical head
device, incident light is effectively converged on an information
recording plane of optical disks for HD and DVD.
[0177] Further, after multi-layer film filters having the same
construction are formed in the annular region (A.sub.1) and the
annular region (A.sub.2), the diffraction grating 91 may be formed
only in the annular region (A.sub.1).
[0178] Further, the step for phase adjustment is formed by
processing the surface in the annular region (A.sub.2) of the
transparent substrate 5 in the above embodiment. However, the step
for the phase adjustment may be formed by depositing a transparent
dielectric film only in the annular region (A.sub.1) and the
circular region (A.sub.3) on the surface of the transparent
substrate 5 so as to satisfy d.sub.1=d.sub.3.
[0179] Here, the diffraction grating 91 is formed on the
transparent dielectric film in the annular region (A.sub.1) so as
to form convex portions having a refractive index of n with a
concavo-convex depth of (d.sub.4-d.sub.1)=.lamda..sub.1/(n-1).
[0180] Here, when a transparent dielectric film for phase
adjustment is deposited in the circular region (A.sub.3), it is
preferred to employ a single layer of intermediate refractive index
dielectric material made of a mixture or a compound of a
low-refractive index dielectric material and a high-refractive
index dielectric material. By adjusting the refractive index of the
intermediate refractive index dielectric material, the thicknesses
of the transparent dielectric film and the multi-layer film filter
92 can be made equal (d.sub.2=d.sub.3), and accordingly, e.g. a
phase correction layer can further be formed on the surface.
[0181] Here, the plan pattern of the diffraction grating 91 is
designed not to have a twice rotational symmetry with respect to
the optical axis of a transmitted light. For example, in FIG. 12,
two-divided grating patterns being symmetrical with respect to Y
axis are formed, and they do not have a twice rotational symmetry
with respect to the optical axis. In the case of the two-divided
grating patterns, the two-divided grating patterns may have a
concentric circular form or a form in which the grating pitch is
not even.
[0182] By forming such a grating pattern, light diffracted by the
diffraction grating 91 in the incoming path and reflected by the
information recording plane of an optical disk, is diffracted again
by the diffraction grating 91 in the returning path, whereby it is
possible to prevent it from going through the same optical path as
a signal light of recorded information of the optical disk and
being incident in the photo-acceptance surface of a photodetector.
As a result, an aperture-limiting function with a wavelength
selectivity of the annular region (A.sub.1) can be substantially
obtained.
[0183] Further, FIG. 14 shows a cross-sectional view of a phase
correction element 800 according to a modified example of the
seventh embodiment, wherein instead of the multi-layer film filter
92 formed in the annular region (A.sub.2), an aperture-limiting
substrate 5B in which a diffraction grating 93 having a
wavelength-selectivity of transmitting light having a wavelength of
.lamda..sub.1 and that having a wavelength of .lamda..sub.2 and
diffracting light having a wavelength of .lamda..sub.3, is
employed.
[0184] By forming the diffraction grating 93 having a
concavo-convex form in cross-section with a ratio between the
lengths of the concave portion and the convex portion of 1:1 and
the function for generating a wavelength phase difference of about
5 times of the wavelength .lamda..sub.1, the wavelength phase
difference becomes about 3 times of wavelength .lamda..sub.2 and
about 2.5 times of wavelength .lamda..sub.3, whereby an
aperture-limiting function by which incident light having a
wavelength of .lamda..sub.1 and incident light having a wavelength
.lamda..sub.2 are straightly transmitted and incident light having
a wavelength of .lamda..sub.3 is diffracted so that at most 30% of
the light is transmitted straightly is provided.
[0185] The same wavelength-selection function is exhibited if the
wavelength phase difference between the concave portion and the
convex portion is substantially an integer number times of
wavelength .lamda..sub.1 and wavelength .lamda..sub.2 and
non-integer times, preferably a value close to an odd number times
of 1/2, of wavelength .lamda..sub.3.
[0186] A wavefront aberration is not generated if the phase
differences of transmitted light having a wavelength of
.lamda..sub.1 and that having a wavelength .lamda..sub.2 between
the convex portion of the diffraction grating 93 and the circular
region (A.sub.3), are integer number times of wavelength
.lamda..sub.1 and wavelength .lamda..sub.2 respectively. As shown
in FIG. 14, by directly processing the surface of the transparent
substrate 5 to form the diffraction grating 91 in the annular
region (A.sub.1) and the diffraction grating 93 in the annular
region (A.sub.2) so that the surfaces of the convex portions of the
grating are in phase, transmitted wavefronts of transmitted light
having a wavelength of .lamda..sub.1 through the NA.sub.1 region in
the aperture-limiting substrate 5B and transmitted light having a
wavelength of .lamda..sub.2 through the NA.sub.2 region are not
changed, and no wavefront aberration is generated.
[0187] The phase correction elements 700 and 800 show cases where
two types of aperture-limiting functions having different
wavelength-selectivity are formed in the annular region (A.sub.1)
and the annular region (A.sub.2) respectively. However, in a case
where the first phase correction layer has a function of limiting
the numerical aperture to be NA.sub.2 for a light having a
wavelength of .lamda..sub.2, the multi-layer film filter 92 or the
diffraction grating 93 is formed only in the annular region
(A.sub.2) or both in the annular region (A.sub.1) and the annular
region (A.sub.2) Further, an example where an aperture-limiting
means is formed on the surface of the transparent substrate 5, has
been shown. However, it may be formed on the surface of the
transparent substrate 6 or in the phase correction element.
[0188] In the phase correction element 700, an aperture-limiting
function can be obtained by forming a rectangular diffraction
grating having a rectangular shape in cross section and having a
fine diffraction pitch, that is made of the same transparent
material 1A as that of the first phase correction layer 10B formed
in the region of numerical aperture NA.sub.2 for the circular
region (NA.sub.1), to diffract incident light having a wavelength
of .lamda..sub.2 and incident light having a wavelength of
.lamda..sub.3. In this case, the height of the convex portion of
the rectangular diffraction grating has to be made about a half of
that of the transparent material 1A of the saw-tooth-like grating
of the phase correction layer 10B.
[0189] Further, in the phase correction elements 300 and 500, an
aperture-limiting function can be obtained by forming a rectangular
diffraction grating having a rectangular shape in cross section,
having a fine diffraction pitch and having a small grating pitch in
the same manner as the first phase correction layers 10C and 10E
formed in the region of numerical aperture NA.sub.2, also in the
annular region (A.sub.1) on the surface of the transparent
substrate 5, to diffract incident light having a wavelength of
.lamda..sub.2 and incident light having a wavelength of
.lamda..sub.3. In this case, the height of the convex portion of
the rectangular diffraction grating has to be the height d.sub.N1
of one step of the phase correction layers 10C and 10E as step-like
gratings.
[0190] Further, in the phase correction element 500, an
aperture-limiting function can be obtained by forming a rectangular
diffraction grating having a rectangular shape in cross section and
fine grating pitch, made of the same high-molecular liquid crystal
as the second phase correction layer 20E formed in the region of
numerical aperture NA.sub.3, also in the annular region (A.sub.1)
and the annular region (A.sub.2), to diffract an extraordinarily
polarized incident light having a wavelength of .lamda..sub.3. In
this case, the height of the convex portion of the rectangular
diffraction grating have to be the height d.sub.M1 of one step of
the phase correction layer 20E as a step-like grating.
Eighth Embodiment
[0191] An example of the optical head device provided with the
phase correction element according to the present invention
obtained from the first to seventh embodiments, will be described
using FIGS. 15 to 18.
[0192] FIG. 15 is a construction view showing an optical head
device provided with the first phase correction element 100
according to the first embodiment. Here, the phase correction
element 100 is not limited to the above-mentioned first phase
correction element 100, but may be any one of the first to seventh
phase correction elements.
[0193] Further, in FIGS. 16 to 18, (a), (b) and (c) are
cross-sectional views showing light beams and wavefronts when three
types of light having wavelengths of .lamda..sub.1, .lamda..sub.2
and .lamda..sub.3 are incident into the phase correction element
respectively. FIG. 16 shows a case of the seventh phase correction
element 700, FIG. 17 shows a case of the fifth phase correction
element 500 and FIG. 18 shows a case of a phase correction element
900 which is the sixth phase correction element 600 in which a
multi-layer film filter (not show) is formed in the annular region
(A.sub.1) and the annular region (A.sub.2) to transmit light having
a wavelength of .lamda..sub.1 and light having a wavelength of
.lamda..sub.2 and to reflect light having a wavelength of
.lamda..sub.3.
[0194] The optical head device is provided with a semiconductor
laser 14A emitting a light in a .lamda..sub.1=410 nm wavelength
band used for an optical disk for HD, a semiconductor laser 14B
generating a light in a .lamda..sub.2=650 nm wavelength band to be
used for an optical disk for DVD, and a semiconductor laser 14C
generating a light in a .lamda..sub.3=780 nm wavelength band to be
used for an optical disk for CD, and provided with a photodetector
15A receiving light having a wavelength of .lamda..sub.1, a
photodetector 15B receiving light having a wavelength of
.lamda..sub.2, and an photodetector 15C receiving light having a
wavelength of .lamda..sub.3.
[0195] Further, in this optical head device, a polarizing beam
splitter 19, a light-combiner prism 17, a collimator lens 13, the
above-mentioned phase correction element 100 and an objective lens
12 are disposed in the optical path of light having a wavelength of
.lamda..sub.1.
[0196] Further, in this optical head device, a hologram beam
splitter 16B and a light-combiner prism 18 are disposed in the
optical path of light having a wavelength of .lamda..sub.2 to lead
the light to the light-combiner prism 17, and a hologram beam
splitter 16C is disposed in the optical path of light having a
wavelength of .lamda..sub.3 to lead the light to the light-combiner
prism 18.
[0197] (I) In the above construction, the light having a wavelength
of .lamda..sub.1 emitted from the semiconductor laser 14A is
reflected by the polarizing beam splitter 19, transmitted through
the light-combiner prism 17, and turned into a parallel light by
the collimator lens 13 and incident in the phase correction element
100 as an ordinarily polarized light. Further, it is transformed
into a circularly polarized light by the first phase plate in the
phase correction element 100 functioning as a 1/4 phase plate to
light having a wavelength of .lamda..sub.1, and straightly
transmitted through the phase correction element 100 as shown in
(a) of FIGS. 16 to 18. Then, light beam corresponding to a
numerical aperture NA.sub.1=0.85 is converged on an information
recording medium of an optical disk D.sub.1 (an optical disk for
HD) by the objective lens 12 designed so as to correspond to the
optical disk D.sub.1 for HD.
[0198] A signal light having a wavelength of .lamda..sub.1
reflected by the information recording plane of the optical disk
D.sub.1, goes back through the incoming path and transformed into
extraordinarily polarized light by the first phase plate in the
phase correction element 100, straightly transmitted through the
phase correction element 100, transmitted through the
light-combiner prism 17 and the polarizing beam splitter 19 and
effectively converged on a photo-receiving surface of the
photodetector 15A to be transformed into an electrical signal.
[0199] (II) Further, at least a half of light having a wavelength
of .lamda..sub.2 emitted from the semiconductor laser 14B, is
transmitted through the hologram beam splitter 16B, transmitted
through the light-combiner prism 18, reflected by the
light-combiner prism 17 and converged by a collimator lens 13 to be
a parallel light and incident in the phase correction element 100.
Further, in the phase correction element 100, the transmitted
wavefront of the light beam corresponding to a numerical aperture
NA.sub.2=0.60 is transformed as shown in (b) of FIGS. 16 to 18 by
the first phase correction layer in the phase correction element
100 so that the wavefront aberration caused by the difference of
cover thickness of optical disks is corrected and the power
corresponding to the concave lens is imparted. Then, the light beam
transmitted through the phase correction element 100 is converged
on an information recording plane of an optical disk D.sub.2 (in
this case, an optical disk for DVD) by the objective lens 12.
[0200] Signal light having a wavelength of .lamda..sub.2 reflected
by the information recording plane of the optical disk D.sub.2,
goes back through the incoming path, a part of the signal light is
diffracted by the hologram beam splitter 16B, and converged on an
photo-acceptance surface of the photodetector 15B, and transformed
into an electric signal.
[0201] (III) Further, with respect to the light having a wavelength
of .lamda..sub.3 emitted from the semiconductor laser 14C, at least
a half of the light is transmitted through the hologram beam
splitter 16C, reflected by the light-combiner prism 18 and the
light-combiner prism 17, and converged by a collimator lens 13 to
be substantially parallel light and incident into the phase
correction element 100. Further, the transmitted wavefront of the
light beam corresponding to a numerical aperture NA.sub.3=0.45 in
the phase correction element 100, is transformed as shown in (c) of
FIGS. 16 to 18 by the first or second phase correction layer in the
phase correction element so as to correct the wavefront aberration
caused by the difference of cover thicknesses of optical disks and
to impart the power corresponding to the concave lens. Then, the
light beam transmitted through the phase correction element 100 are
converged on the information recording plane of an optical disk
D.sub.3 (in this case, an optical disk for CD) by the objective
lens 12.
[0202] Signal light having a wavelength of .lamda..sub.3 reflected
by the information recording plane of the optical disk D.sub.3,
goes back through the incoming path and a part of the light is
diffracted by the hologram beam splitter 16C, and converged on an
photo-receiving surface of the photodetector 15C to be transformed
into an electrical signal.
[0203] In the phase correction element 700 shown in FIG. 11, since
a first phase correction layer 10G having no polarization
dependency is used, aberration correction can be performed
regardless of the polarization state of incident light having a
wavelength of .lamda..sub.2. Further, by employing a first phase
plate 30D (refer to FIG. 6) functioning as a 1/4 waveplate for
three types of incident light having a wavelength of .lamda..sub.1,
a wavelength of .lamda..sub.2 and a wavelength of .lamda..sub.3
respectively, it is possible to transform light coming and
returning in the phase plate into linearly polarized light having a
polarization plane perpendicular to that of the incident light.
[0204] Further, by employing a polarizing hologram beam splitter
transmitting ordinarily polarized light and diffracting
extraordinarily polarized light as the hologram beam splitter 16B
or 16C, light utilization efficiency can be improved. Or, since
light having the same polarization plane as that of the emitted
light, does not return to the emission point of the semiconductor
laser, the laser emission is stabilized and recording and/or
reproducing can be performed with high reliability.
[0205] In the phase correction element 500 shown in FIG. 7, when
the light having a wavelength of .lamda..sub.2 is made an
ordinarily polarized light in the incoming and returning paths, the
correction of the wavefront aberration of an incident light having
a wavelength of .lamda..sub.2 can be achieved by the first phase
correction layer 10E.
[0206] Further, in the phase correction element 900 shown in FIG.
18, when the light having a wavelength of .lamda..sub.2 is made to
be extraordinarily polarized light in the incoming and returning
paths, the correction of the wavefront aberration of incident light
having a wavelength of .lamda..sub.2 can be achieved by the second
polarizing phase correction layer 10F.sub.2 (refer to FIG. 10).
[0207] In the phase correction element 500 shown in FIG. 7, when
the light having a wavelength of .lamda..sub.3 is made to be an
extraordinarily polarized light in the incoming and returning
paths, the correction of the wavefront aberration of an incident
light having a wavelength of .lamda..sub.3 can be achieved by the
second phase correction layer 20E.
[0208] Further, in the phase correction element 700 shown in FIG.
11 and in the phase correction element 900 shown in FIG. 18, an
aberration correction function is generated also for an incident
light having a wavelength of .lamda..sub.3 by the first phase
correction layer 10G or the second polarization phase correction
layer 10F.sub.2 (refer to FIG. 10) in the incoming and returning
paths. However, since a spherical aberration remains, a good
wavefront aberration correction can be performed by making the
incident light having a wavelength of .lamda..sub.3 to be a slight
divergent light with respect to the phase correction element and
the objective lens. Or, by making the incident light having a
wavelength of .lamda..sub.2 and incident light having a wavelength
of .lamda..sub.3 to have the same degree of divergence with respect
to the phase correction element and the objective lens, the phase
correction layer can formed so that a wavefront aberration
correction can be performed to lights of both wavelengths. Here, in
the phase correction element 900, the incident light of wavelength
of .lamda..sub.3 is made to be the same extraordinarily polarized
light as the incident light of wavelength of .lamda..sub.2, so that
its aberration is corrected by the second polarization phase
correction layer 10F.sub.2.
[0209] Further, in a case where an objective lens designed to be
adapted for an optical disk for HD in a .lamda..sub.1=410 nm
wavelength band and having a cover thickness of 0.1 mm, is used for
optical disks for DVD and CD, in order to maintain the distance
between the objective lens and the optical disks, it is preferred
that the phase correction layer has a grating pattern which
produces a transmitted wavefront having a power component
functioning as a concave lens in addition to a spherical aberration
correction component.
[0210] Here, the phase correction element 700 shown in FIG. 11
shows a case where the diffraction grating 91 limits incident light
having a wavelength of .lamda..sub.2 within a numerical aperture of
NA.sub.2=0.60, and the diffraction grating 91 and the multi-layer
film filter 92 limit incident light having a wavelength of
.lamda..sub.3 within a numerical aperture of NA.sub.3=0.45.
[0211] Further, the phase correction element 500 shown in FIG. 7
shows a case where the first phase correction layer 10E limits
incident light having a wavelength of .lamda..sub.2 within a
numerical aperture of NA.sub.2=0.60, and the second phase
correction layer 20E limits incident light having a wavelength of
.lamda..sub.3 within a numerical aperture of NA.sub.3=0.45.
[0212] On the other hand, the phase correction element 900 shown in
FIG. 18 shows a case where the second polarization phase correction
layer 10F.sub.2 (refer to FIG. 10) limits the incident light having
a wavelength of .lamda..sub.2 within a numerical aperture of
NA.sub.2=0.60, and the multi-layer film filter 92 (refer to FIG.
11, here, it is formed in the annular regions (A.sub.1) and
(A.sub.2)) limits the incident light having a wavelength of
.lamda..sub.3 within a numerical aperture of NA.sub.3=0.45.
[0213] Therefore, by employing any one of the first to eighth phase
correction elements shown in the embodiments of the present
invention, a wavefront aberration generated when the objective lens
12 designed for an optical disk for HD having a cover thickness of
0.1 mm, is used for recording and/or reproducing an information in
an optical disk for DVD having a cover thickness of 0.6 mm or in an
optical disk for CD having a cover thickness of 1.2 mm, can be
corrected. Therefore, it is possible to stably converge light
transmitted from a semiconductor laser to an information recording
plane of an optical disk, and to achieve recording and/or
reproducing an information in three types of optical disks of HD,
DVD and CD.
[0214] Further, as shown in FIGS. 16 to 18, by forming the phase
correction layer so that light having a wavelength of .lamda..sub.2
and light having a wavelength of .lamda..sub.3 are each converted
to be divergent light after they are transmitted through the phase
correction element, the distance (working distance) between the
objective lens 12 and optical disks D.sub.2 and D.sub.3 for DVD and
CD can be extended as shown by the dotted lines for the optical
path in FIG. 15. By making the working distance large, the
stability when the objective lens 12 and the phase correction
element (100 to 900) are provided on an actuator (not shown) and
the focus servo is actuated, is improved.
[0215] Here, in the above-mentioned embodiment, the objective lens
12 for HD designed to have a numerical aperture NA.sub.1=0.85
corresponding to an optical disk D.sub.1 for HD in a
.lamda..sub.1=410 nm wavelength band having a cover thickness of
0.1 mm is assumed, it is also possible to provide a phase
correction element necessary for recording and/or reproducing
information in an optical disk D.sub.2 for DVD and an optical disk
D.sub.3 for CD, employing an objective lens designed to have a
numerical aperture of about NA.sub.1=0.65 corresponding to an
optical disk D.sub.1 for HD having a cover thickness of 0.6 mm.
[0216] In this case, since the optical disks D1 and D2 for HD and
DVD respectively have about the same cover thickness, and objective
lenses for these disks have about the same numerical aperture, a
spherical aberration generated by a refractive index wavelength
dispersion of the optical material (particularly for the objective
lens) caused by the difference of wavelength to be used, can be
corrected. Further, by making the numerical apertures NA.sub.1 and
NA.sub.2 for a .lamda..sub.1=410 nm wavelength band and a
.lamda..sub.2=650 nm wavelength band, respectively, to be about the
same, the aperture-limiting function can be provided by any one of
the multi-layer film filter 92 or the diffraction grating 93 which
transmits light having a wavelength of .lamda..sub.1 and light
having a wavelength of .lamda..sub.2 and does not transmits light
in a .lamda..sub.3=780 nm wavelength band.
[0217] Further, since the numerical aperture for HD is about the
same as the numerical aperture for DVD, it is not necessary to
extend the distance between the objective lens and optical disks
for DVD and CD. Therefore, the phase correction element does not
need to have function as a concave lens producing a transmitted
wavefront having a large power component, and only the spherical
aberration component be converted.
[0218] In a case of correcting a spherical aberration generated
when the objective lens for HD having numerical aperture of about
NA.sub.1=0.65 is used for an optical disk D.sub.2 for DVD with a
wavelength of .lamda..sub.2 and a numerical aperture NA.sub.2, by
the phase correction element, the phase correction layer to be
fabricated is the same as the first phase correction layer 10C
employed in the third phase correction element 300 shown in FIG.
4.
[0219] The phase correction element 300 has a Fresnel lens form
constituted by a blazed diffraction grating having a multi-step
form in cross section in order to correct a spherical aberration
containing a large power component shown in (B) of FIG. 3. However,
also in a case where the wavefront aberration to be corrected is a
spherical aberration and the size is within one wavelength, the
multi-step-like pattern has to be determined by the wavefront
aberration correction method shown in FIG. 5.
[0220] Here, in a case of correcting a spherical aberration by a
step-like pattern, a high-order aberration component remains in the
transmitted wavefront. Such high-order aberration can be reduced by
processing the refractive index wavelength dispersion material
constituting the first phase correction layer 10B employed in the
second phase correction element 200 (refer to FIG. 1) to have a
shape by which the high-order wavefront aberration can be
corrected, and by using the first phase correction layer 10C (refer
to FIG. 4) in combination with it. Further, such high-order
aberration can be reduced by processing the high-molecular liquid
crystal constituting the polarizing phase correction layer employed
in the sixth phase correction element 600 shown in FIG. 10, to have
a shape by which the high-order wavefront aberration can be
corrected, and by using the first phase correction layer 10C in
combination with it.
[0221] The method for correcting a spherical aberration generated
when the objective lens for HD having a numerical aperture of about
NA.sub.1=0.65, is used for an optical disk D.sub.3 for CD with a
wavelength of .lamda..sub.3 and a numerical aperture of NA.sub.3,
may be a method for reforming incident light into divergent light
to the objective lens or a method for employing the same phase
correction layer as the second phase correction layer 20E employed
in the fifth phase correction element 500 shown in FIG. 7. Here, as
aperture limitation for limiting light beams having a wavelength of
.lamda..sub.3 to have a numerical aperture of NA.sub.3, a
multi-layer film filter 92 employed in the seventh phase correction
element 700 shown in FIG. 11 or the diffraction grating 93 employed
in the eighth phase correction element 800 shown in FIG. 14, should
be formed in an outer area of the numerical aperture NA.sub.3.
[0222] Now, Examples will be described.
EXAMPLE 1
[0223] An example of the seventh phase correction element 700
according to the present invention will be described using FIG. 11
(cross-sectional view) and FIG. 12 (plan view).
[0224] In order to produce the seventh phase correction element
700:
[0225] (1) First of all, a diffraction grating 91 having a
concavo-convex portion in cross section in which the depth of is
862 nm and the ratio in length of a concave portion to a convex
portion is 1:1, and having a linear plan shape, is formed in an
annular region (A.sub.1) obtained by subtracting a circular region
of numerical aperture NA.sub.2=0.60 from a circular region of
numerical aperture NA.sub.1=0.85 of a surface of a glass substrate
(transparent substrate) having a refractive index=1.47 by etching
process to form two divided patterns symmetrical to each other with
respect to Y axis, the patterns being inclined at an angle of
.+-.45.degree. to Y axis.
[0226] (2) Then, a four-layered antireflective film 8 (refer to
FIG. 13) made of SiO.sub.2 and TiO.sub.2 as an antireflective film
8 for three types of light in a .lamda..sub.1=410 nm wavelength
band, in a .lamda..sub.2=650 nm wavelength band and a
.lamda..sub.3=780 nm wavelength band respectively, is deposited in
the entire area of the surface on which the diffraction grating 91
is formed. The construction is shown in Table 1. In this case, the
optical path length (refractive index.times.film thickness) of the
antireflective film 8 is 331 nm at a wavelength of .lamda..sub.1
and 322 nm at a wavelength of .lamda..sub.2. TABLE-US-00001 TABLE 1
Refractive index (wavelength) Film Layer Material (405 nm) (660 nm)
thickness Ambience Air 1.0 1.0 1 SiO.sub.2 1.470 1.456 123.9 nm 2
TiO.sub.2 2.530 2.271 11.4 nm 3 SiO.sub.2 1.470 1.456 57.8 nm 4
TiO.sub.2 2.530 2.271 14.0 nm Substrate Quartz 1.470 1.456
[0227] TABLE-US-00002 TABLE 2 Refractive index (wavelength) Film
Layer Material (405 nm) (660 nm) thickness Ambience Air 1.0 1.0 1
SiO.sub.2 1.470 1.456 73.1 nm 2 Ta.sub.2O.sub.5 2.195 2.131 103.3
nm 3 SiO.sub.2 1.470 1.456 121.3 nm 4 Ta.sub.2O.sub.5 2.195 2.131
89.5 nm 5 SiO.sub.2 1.470 1.456 162.6 nm 6 Ta.sub.2O.sub.5 2.195
2.131 98.8 nm 7 SiO.sub.2 1.470 1.456 123.9 nm 8 Ta.sub.2O.sub.5
2.195 2.131 89.4 nm 9 SiO.sub.2 1.470 1.456 141.8 nm 10
Ta.sub.2O.sub.5 2.195 2.131 94.0 nm 11 SiO.sub.2 1.470 1.456 169.2
nm 12 Ta.sub.2O.sub.5 2.195 2.131 97.5 nm Substrate Quartz 1.470
1.456
[0228] (3) Then, a transparent dielectric film Ta.sub.2O.sub.5
having a high refractive index and a transparent dielectric film
SiO.sub.2 having a low refractive index are laminated alternately
to form a 12-layer lamination in an annular region (A.sub.2)
obtained by subtracting a circular region of numerical aperture
NA.sub.3 from a circular region of numerical aperture NA.sub.2=0.6
in the surface of the glass substrate 5 (transparent substrate 5),
to form a multi-layer film filter 92 transmitting at least 90% of
light having a wavelength of .lamda..sub.1 and light having a
wavelength of .lamda..sub.2 and reflecting at least 70% of light
having a wavelength of .lamda..sub.3. The construction is shown in
Table 2. Here, the total film thickness d.sub.2 of the multi-layer
film filter 92 is 1,364 nm, the optical path length is 2,420 nm at
a wavelength of .lamda..sub.1 and 2,373 nm at a wavelength of
.lamda..sub.2.
[0229] When such multi-layer film filter 92 is formed in the
annular region (A.sub.2) of the glass substrate 5, the annular
region (A.sub.2) of the glass substrate 5 is subjected to an
etching process to form a step for phase adjustment in advance
before depositing the multi-layer film filter 92 so as not to
generate a phase difference with respect to the circular region
(A.sub.3). Specifically, the glass substrate 5 is processed to have
a depth of 2,187 nm with respect to the surface S3 of the circular
region (A.sub.3) of FIG. 13 including the thickness 207 nm of the
antireflective film 8.
[0230] Here, the optical path length (L.sub.3) between S.sub.0
surface and S.sub.3 surface in the circular region (A.sub.3) for
incident light having a wavelength of .lamda..sub.3, is represented
by the formula (3), and is the sum of the optical path length of
the glass substrate 5 after the etching process and the
antireflective film 8, which is L.sub.3=3,242 nm.
[0231] On the other hand, the optical path length (L.sub.2) between
S.sub.0 surface and S.sub.3 surface in the annular region
(A.sub.2), is calculated by formula 2, and is the sum of the
optical path length 2,420 nm (=1.774.times.1,364 nm) of the
multi-layer film filter 92 and the optical path length 823 nm of
air layer between S.sub.2 surface and S.sub.3 surface generated in
the space to the circular region (A.sub.3), which is L.sub.2=3,243
nm and approximately L.sub.2=L.sub.3. Therefore, no phase
difference is generated in the annular region (A.sub.1) of the
aperture-limiting substrate 5A and the circular region (A.sub.3)
for incident light having a wavelength of .lamda..sub.1.
Accordingly, the same transmitted wavefront as that of the incident
light is obtained in the entire region of aperture NA.sub.1.
[0232] Further, also, for incident light having a wavelength of
.lamda..sub.2, the optical path length L.sub.3 between S.sub.0
surface and S.sub.3 surface in the circular region (A.sub.3) is
L.sub.3=3,197 nm, and the optical path length L.sub.2 between
S.sub.0 surface and S.sub.3 surface in the circular region
(A.sub.2) is L.sub.2=3,205 nm, and approximately L.sub.2=L.sub.3.
Therefore, also for incident light having a wavelength of
.lamda..sub.2, the same transmitted wavelength as that of the
incident light can be obtained in the entire region of numerical
aperture NA.sub.2 of the aperture-limiting substrate 5A.
[0233] The spectral transmittance of the aperture-limiting
substrate 5A of this Example thus obtained is shown in FIG. 19. The
spectral transmittance of the diffraction grating 91 formed in the
annular region (A.sub.1) is shown by a line (a), the spectral
transmittance of the multi-layer film filter 92 formed in the
circular region (A.sub.2) is shown by a line (b), and the spectral
transmittance of the antireflective film 8 formed in the circular
region (A.sub.3) is shown by a line (c).
[0234] (4) Then, in the opposite surface of the glass substrate 5
from the surface on which the aperture-limiting function is formed,
an SiON film made of a mixed composition of SiN and SiO.sub.2 as a
transparent material of an optical index n.sub.A, is deposited to
have a film thickness of 32 .mu.m. Then, it is processed to have a
Fresnel lens shape having a saw-tooth-form in cross section as
shown in FIG. 11, to make it to be a transparent substrate 1A
constituting the first phase correction layer 10G. Further, a
high-refractive index transparent plastic material as a transparent
material 1B having a refractive index of n.sub.B is filled and
solidified in the concave portion to form a first phase correction
layer 10G.
[0235] Here, the refractive indexes of the transparent material 1A
(SiON) and the transparent material 1B (high-refractive index
plastic material), are substantially equal to each other at a
wavelength of .lamda..sub.1, a refractive index difference of 0.020
is generated at a wavelength of .lamda..sub.2 and a refractive
index difference of 0.023 is generated at a wavelength of
.lamda..sub.3. Therefore, the first phase correction layer 10G does
not change the transmitted wavefront for incident light having a
wavelength of .lamda..sub.1, but changes wavefronts of incident
light having a wavelength of .lamda..sub.2 and incident light
having a wavelength of .lamda..sub.3. Since the step height of the
concavo-convex portion having a saw-tooth-form corresponds to the
phase difference of about 1 wavelength for a middle wavelength such
as the wavelength .lamda..sub.2 and the wavelength .lamda..sub.3,
the first-order diffraction light is maximized at the wavelength
.lamda..sub.2 and the wavelength of .lamda..sub.3.
[0236] Here, in the first phase correction layer 10G, the
transparent material 1A has a larger refractive index than the
transparent material 1B at the wavelength .lamda..sub.2 and the
wavelength .lamda..sub.3, the first phase correction layer 10G has
the cross-sectional shape as shown in FIG. 11.
[0237] (5) Then, after coating on one side of the glass substrate 6
with polyimide and applying a alignment treatment in a direction at
an angle of 163.degree. to X axis, the glass substrate 6 is further
coated with a solution of an acrylic type liquid crystal monomer,
irradiated with an ultraviolet light to polymerize and cure the
liquid crystal monomer to form a phase plate 3B comprising a
high-molecular liquid crystal film of a birefringent material
having a fast axis aligned to be at an angle of 73.degree. to X
axis. Here, the retardation value of the phase plate 3B at a
wavelength of 520 nm corresponding to the middle wavelength between
the wavelength .lamda..sub.2 and the wavelength .lamda..sub.3, is
130 nm which corresponds to about 1/4 wavelength. Here, with
respect to the angle, the positive angle indicates an angle in
counterclockwise direction in FIG. 12.
[0238] Further, by employing an organic thin film made of
polycarbonate exhibiting birefringency by drawing, for the phase
plate 3A, it is laminated on the phase plate 3B using an adhesive,
and is bonded and fixed to the phase correction layer 1. The
retardation value of the phase plate 3A at a wavelength of 520 nm
is 260 nm which corresponds to about 1/2 wavelength, and the fast
axis extends in a direction at an angle of 17.degree. to X axis.
Namely, the fast axes of the phase plate 3A and the phase plate 3B
are at an angle of 56.degree. to each other.
[0239] Thus, the first phase plate 30G is produced by laminating
the phase plate 3A and the phase plate 3B.
[0240] According to the seventh phase correction element 700 thus
produced, when three types of light of wavelengths of
.lamda..sub.1, .lamda..sub.2 and .lamda..sub.3 respectively, having
polarization planes in the X axis direction, are incident into the
first phase plate 30G from the side of the aperture-limiting
substrate 5A, every light is turned into circularly polarized light
having a ellipticity .kappa. of at least 0.9 to be emitted, whereby
the function corresponding to a 1/4 waveplate can be obtained for
these three wavelengths.
EXAMPLE 2
[0241] Next, an example of the fifth phase correction element 500
according to the present invention will be described using FIG. 7
(cross-sectional view) and FIG. 8 (plan view).
[0242] (1) Etching is carried out directly to an area of numerical
aperture NA.sub.2=0.60 in the surface of a glass substrate 5
(transparent substrate) having refractive indexes at three
wavelengths .lamda..sub.1=405 nm, .lamda..sub.2=655 nm and
.lamda..sub.3=790 nm of 1.470, 1.456 and 1.454 respectively, to
form a first phase correction layer 10E comprising a step-like
blazed diffraction grating having an orbicular zone-shaped
concavo-convex shape (a Fresnel lens shape) with a rotational
symmetry with respect to the optical axis, the cross-sectional
shape of the grating being in a saw-tooth-form approximated by five
levels (four steps) of step.
[0243] Here, the height d.sub.N1 of one step of the step-like
grating is 1.723 .mu.m so that an optical path difference of
2.times..lamda..sub.1 from the air is produced at a wavelength of
.lamda..sub.1. Here, the optical path difference is
1.2.times..lamda..sub.2, namely it corresponds to
0.2.times..lamda..sub.2, at a wavelength of .lamda..sub.2, and the
optical path difference is about .lamda..sub.3 at a wavelength of
.lamda..sub.3. Namely, the transmitted wavefronts of light having a
wavelength of .lamda..sub.1 and light having a wavelength of
.lamda..sub.3 incident into the 5-level (4 steps) step-like
grating, are not changed. However, the transmitted wavefront of a
light having a wavelength of .lamda..sub.2 is changed by the
orbicular zone distribution of the step-like grating.
[0244] The radius of each orbicular zone of step-like grating is
determined so that a transmitted wavefront aberration generated
when an objective lens for HD of NA.sub.1=0.85 designed to have
good aberration for an optical disk for HD having a cover thickness
of 0.1 mm at a wavelength of .lamda..sub.1, is used for an optical
disk for DVD having a cover thickness of 0.6 mm at a wavelength of
.lamda..sub.2 with NA.sub.2=0.60, can be corrected.
[0245] (2) On the opposite surface of the glass substrate 5 from
the surface on which the first phase correction layer 10E is
formed, a polymer liquid crystal layer having an ordinary
refractive index of n.sub.o and an extraordinary refractive index
of n.sub.e having the fast axis aligned to X axis, is formed by the
same process as that of phase plate 3B of Example 1. Further, a
blazed diffraction grating 2A having an orbicular-zone-shaped
concavo-convex form (Fresnel lens form) having a rotational
symmetry with respect to the optical axis, the cross-sectional
shape of the grating of saw-tooth-form being approximated by
3-levels (2 steps) of step, is formed in a region of numerical
aperture NA.sub.3=0.45 by a photolithography process and a reactive
ion etching process, followed by filling the concave portions with
a homogeneous refractive index transparent material 2B having a
refractive index of n.sub.s which is approximately equal to the
ordinary refractive index n.sub.o, whereby the second phase
correction layer 20E is formed.
[0246] Here, the refractive index difference (n.sub.e-n.sub.s) at
the wavelengths .lamda..sub.1, .lamda..sub.2 and .lamda..sub.3 are
0.277, 0.213 and 0.200 respectively, and the height d.sub.M1 of one
step of the step-like grating is made to be 1.462 .mu.m. In this
case, the optical path difference in one step of the step-like
blazed diffraction grating 2A made of the high-molecular liquid
crystal layer from the homogeneous refractive index transparent
material 2B for an extraordinarily polarized light, becomes
.lamda..sub.1 at a wavelength of .lamda..sub.1 and
0.37.times..lamda..sub.3 at a wavelength of .lamda..sub.3.
Therefore, the transmitted wavefront of ordinarily polarized light
incident into the second phase correction layer 20E is not changed
regardless of the wavelength, and the transmitted wavefront having
a wavelength of .lamda..sub.1 is not changed regardless of the
incident polarization. On the other hand, the transmitted wavefront
of extraordinarily polarized incident light having a wavelength of
.lamda..sub.3 is changed according to a distribution of the
orbicular zone of the step-like grating.
[0247] The radius of each orbicular zone of step-like grating is
determined so that the transmitted wavefront aberration generated
when the objective lens is used for an optical disk for CD having a
cover thickness of 1.2 mm at a wavelength of .lamda..sub.3 with
NA.sub.3=0.45, is corrected.
[0248] (3) Then, a phase plate 3D comprising a high-molecular
liquid crystal film having a fast axis aligned in X direction, is
formed in the same manner as the phase plate 3B in Example 1 on one
side of the glass substrate 6. Here, the retardation value of the
phase plate 3D for light having a wavelength of .lamda..sub.1, is
203 nm which corresponds to about 1/2 wavelength. Further, in the
same manner as the phase plate 3A of Example 1, a thin organic film
made of polycarbonate is employed for the phase plate 3C, and it is
laminated on the phase plate 3D using an adhesive, and is bonded
and fixed to the phase correction layer 20E. The retardation value
of the phase plate 3C at a wavelength of .lamda..sub.1 is 102 nm
which corresponds to about 1/4 wavelength, and the fast axis is at
an angle of 45.degree. to X axis. In this manner, a first phase
plate 30E having the phase plate 3C and the phase plate 3D
laminated, is produced.
[0249] According to the fifth phase correction element 500 thus
produced, when light having a wavelength of .lamda..sub.1 having a
polarization plane in X axis direction, is incident from the side
of the transparent substrate 5, it is transformed into circularly
polarized light having a ellipticity .kappa. of at least 0.9 and
then the light is emitted, whereby a function corresponding to that
of 1/4 waveplate can be obtained. Further, when light having a
wavelength of .lamda..sub.2 and light having a wavelength of
.lamda..sub.3 having polarization planes in Y axis direction, are
incident, they are each transformed into linearly polarized light
having a polarized plane rotated, whereby a function corresponding
to a 1/2 phase plate can be obtained.
[0250] Therefore, when ordinarily polarized incident light having a
wavelength of .lamda..sub.2 come and returns through the first
phase plate 30E, it becomes the original ordinarily polarized
light, and is straightly transmitted through the second phase
correction layer 20E without changing the transmitted wavefront in
the returning path. Further, when extraordinarily polarized
incident light having a wavelength of .lamda..sub.3 come and
returns through the first phase plate 30E, it becomes the original
extraordinarily polarized light, and the transmitted wavefront is
changed by the second phase correction layer 20E in the returning
path. Thus, a wavefront aberration correction function is
exhibited.
[0251] Here, since ordinarily polarized light in the region of
NA.sub.2 having a wavelength of .lamda..sub.2 transmitted through
the first phase correction layer 10E, and extraordinarily polarized
light in the region of NA.sub.3 having a wavelength of
.lamda..sub.3 transmitted through the second phase correction layer
20E, have transmitted wavefronts containing power components, their
focal planes are different from that of transmitted light other
than the predetermined numerical aperture regions. Therefore, there
is no need to provide an aperture-limiting function in the area
outside the numerical aperture regions.
EXAMPLE 3
[0252] An example of the sixth phase correction element 600
according to the present invention will be described using FIG. 10
(cross-sectional view).
[0253] (1) In an annular region (A.sub.1) and an annular region
(A.sub.2) of a glass substrate 51 (transparent substrate), a
multi-layer film filter (not shown) which is the same as the
multi-layer film filter 92 of Example 1, is formed.
[0254] Further, a first phase plate 30F formed by laminating the
phase plate 3D and the phase plate 3C on one side of the glass
substrate 6 (transparent substrate), is the same as the first phase
plate 30E (FIG. 7) employed in Example 2. Therefore, its
description is omitted.
[0255] (2) On one side of a glass substrate 52, a phase plate 3F
comprising a high-molecular liquid crystal film having a fast axis
aligned at an angle of 70.degree. to X axis, is formed by the same
method as the phase plate 3B in Example 1. Here, the retardation
value of the phase plate 3F for a light having a wavelength of
.lamda..sub.1 is 405 nm which corresponds to about one wavelength.
Further, in the same manner as the phase plate 3A in Example 1, an
organic thin film made of polycarbonate as the phase plate 3E is
used and, it is laminated on the phase plate 3F employing a
homogeneous refractive index transparent material 1G as an
adhesive, and concave portions of the blazed diffraction grating 1F
made of a high-molecular liquid crystal of the second polarizing
phase correction layer 10F.sub.2 are filled with the homogeneous
refractive index transparent material 1G, to bond and fix the phase
plate 3E.
[0256] The retardation value of the phase plate 3E at a wavelength
of .lamda..sub.1 is 405 nm which corresponds to about one
wavelength, and its fast axis is made in a direction at an angle of
25.degree. to X axis. Namely, the fast axes of the phase plate 3E
and the phase plate 3F are at an angle of 45.degree. to each other.
The second phase plate 40F comprising the phase plate 3E and the
phase plate 3F laminated, is produced in this manner. When light
having a wavelength of .lamda..sub.1 having a polarization plane in
X direction, is incident into the second phase plate 40F, the
polarization of emitted light is not changed. Further, when light
having a wavelength of .lamda..sub.2 and light having a wavelength
of .lamda..sub.3 each having a polarization plane in X axis
direction, are incident, they are each transformed into linearly
polarized light having a polarization plane rotated by about
90.degree., whereby a function corresponding to a 1/2 waveplate can
be obtained.
[0257] The second polarizing phase correction layer 10F.sub.2 and
the first polarizing phase correction layer 10F.sub.1 in the sixth
phase correction element 600, are high-molecular liquid crystal
layers formed on one sides of the glass substrates 51 and 52 and
having fast axes aligned in X axis direction, and having an
ordinary refractive index of n.sub.o=1.55 and an extraordinary
refractive index of n.sub.e=1.70, these correction layers being
formed by the same process as that of the second phase correction
layer 20E of Example 2.
[0258] (3) Further, blazed diffraction gratings 1F and 1D each
having an orbicular-zone-like concavo-convex shape (Fresnel lens
shape), having a rotational symmetry with respect to the optical
axis and having a saw-tooth-like cross-sectional shape, are formed
in a region of numerical aperture NA.sub.2=0.60 by a
photolithography process and a reactive ion etching processes, and
the convex portions are respectively filled with homogeneous
refractive index transparent materials 1G and 1E having a
refractive index of n.sub.s substantially equal to the ordinary
refractive index n.sub.o, whereby the second polarizing phase
correction layer 10F.sub.2 and the first polarizing phase
correction layer 10F.sub.1 are formed. Here, as shown in FIG. 10,
the blazed diffraction gratings 1F and 1D are processed so that the
slopes of the saw-tooth-like cross-sections face opposite
directions from each other with respect to the central axis of the
concentric circular grating pattern.
[0259] When ordinarily polarized light is incident into the
saw-tooth-formed blazed diffraction gratings 1D and 1F made of a
high-molecular liquid crystal, the transmitted wavefront does not
change since there is no refractive index difference between the
high-molecular liquid crystal and the homogeneous refractive index
transparent material. On the other hand, when extraordinarily
polarized light is incident, a refractive index difference of about
0.15 is generated between the high-molecular liquid crystal and the
homogeneous refractive index transparent material so that a change
of transmitted wavefront is occurred according to the shape of the
saw-tooth-formed blazed diffraction grating.
[0260] In the second phase correction layer 20E of Example 2, a
step-like blazed diffraction grating 2A of 3 level (2 steps) is
employed. However, the construction of the blazed diffraction
gratings 1D or 1F is different from the diffraction grating 2A in
that the step height of the saw-tooth-like concavo-convex is the
film thickness of the high-molecular liquid crystal corresponding
to a phase difference of about 1 wavelength for extraordinary light
having a wavelength of .lamda..sub.2 and that having a wavelength
of .lamda..sub.3. As a result, the first-order diffraction light is
maximized at the wavelength .lamda..sub.2 and the wavelength
.lamda..sub.3. In this case, the second-order diffraction light is
maximized for an extraordinary light having a wavelength of
.lamda..sub.1.
EXAMPLE 4
[0261] The phase correction element, for example, the sixth phase
correction element 600 thus produced and the objective lens 12 are
integrally formed with an actuator (not shown), and the integrally
formed body is mounted on an optical head device shown in FIG.
15.
[0262] When the optical head device is employed for recording
and/or reproducing optical disks for DVD and CD, a wavefront
aberration generated by an objective lens can be corrected, and the
distance between the objective lens and the optical disk can be
maintained. As a result, the recording and/or reproducing of three
types of optical disks for HD, DVD and CD, can be realized
stably.
[0263] Further, since ordinarily polarized incident light having a
wavelength .lamda..sub.1 is transformed into extraordinarily
polarized outgoing light having a polarization direction
perpendicular to the original direction by the phase plate 30F
while it come and returns, for example, in the sixth phase
correction element 600, a signal light is effectively detected by a
photodetector 15A when a polarizing beam splitter 19 is employed in
combination.
INDUSTRIAL APPLICABILITY
[0264] Since by employing the phase correction element of the
present invention, the transmitted wavefront having a wavelength of
.lamda..sub.2 or a wavelength of .lamda..sub.3 can be corrected
without changing the transmitted wavefront having a wavelength of
.lamda..sub.1. Further, since the first phase plate having a
function of 1/4 waveplate at the wavelength .lamda..sub.1, is
integrally formed, linearly polarized light having a wavelength of
.lamda..sub.1 is transformed into perpendicularly linearly
polarized light as it come and returns through the phase correction
element. As a result, by mounting the phase correction element on
an optical head device in combination with an objective lens for HD
designed to be optimized for an optical disk for HD at a wavelength
of .lamda..sub.1, information in optical disks for HD, DVD and CD
having different wavelengths to be used and cover thicknesses, can
stably be recorded and/or reproduced.
[0265] Further, by employing the optical head device of the present
invention to which a polarizing beam splitter is provided,
light-utilization efficiency of light having a wavelength of
.lamda..sub.1 is improved in the incoming and returning paths, and
the power consumption of a semiconductor laser light source can be
reduced and high speed recording and/or reproducing can be
achieved. Further, since returning light which may cause unstable
emission from the semiconductor laser light source, can be reduced,
the emission of the semiconductor laser is stabilized and an
optical head device for recording and/or reproducing with high
reliability, is realized. Further, since the distance between the
objective lens and the optical disk can be maintained, stability of
e.g. a focus servo at a time of recording and/or reproducing can be
improved. Therefore, an optical head device having an excellent
optical characteristic and being suitable for reducing size and
weight can be provided.
[0266] Further, by employing materials having different refractive
index wavelength dispersions are employed for the first phase
correction layer in the phase correction element, wavefront
aberration correction not relying on the polarization of incident
light, can be performed. Further, by processing the first phase
correction layer in the phase correction element to be a
step-formed grating in which the phase difference of one step
correspond to twice of the wavelength .lamda..sub.1, the wavefront
aberration at a wavelength of .lamda..sub.2 can be independently
corrected.
[0267] Further, by employing a 1/4 waveplate for two wavelengths or
for three wavelengths, an optical head device for recording and/or
reproducing can be easily obtained with high light-utilization
efficiency and high reliability at wavelengths for DVD and CD.
[0268] Further, by employing a material having different
birefringency for the second phase correction layer in the phase
correction element; by forming a step-formed grating in which the
phase difference of one step corresponds to the wavelength
.lamda..sub.1 for an extraordinarily polarized light, and by
specifying the incident polarization of three wavelengths, the
wavefront aberration of the wavelength of .lamda..sub.3 can be
independently corrected. Further, by constituting a phase
correction element of integral structure comprising the first phase
correction layer capable of independently correcting the wavefront
aberration of the wavelength of .lamda..sub.2, and the first phase
plate having a function of 1/4 waveplate at the wavelength of
.lamda..sub.1 and having a function of 1/2 waveplate at the
wavelengths of .lamda..sub.2 and .lamda..sub.3, wavelengths of DVD
and CD can independently be corrected.
[0269] Further, by employing the first phase plate having a
function of 1/4 waveplate at the wavelength of .lamda..sub.1 and
having a function of 1/2 waveplate at the wavelengths of
.lamda..sub.2 and the wavelength of .lamda..sub.3, the second phase
plate having a function of 1/2 waveplate not rotating the
polarization of transmitted light of the wavelength of
.lamda..sub.1 and rotating the polarization planes of the
wavelength of .lamda..sub.2 and the wavelength of .lamda..sub.3 by
90.degree., and two polarizing phase correction layers made of a
birefringent material and a homogeneous refractive index material,
wavefront aberration correction for DVD and CD can be performed. By
employing a high-molecular liquid crystal having a large
birefringency as a birefringent material, the thickness of the
phase correction layer can be reduced and the saw-tooth-like
concavo-convex shape can be formed with good accuracy, whereby
wavefront aberration correction can be performed with high accuracy
and high light-utilization efficiency.
[0270] Further, by forming a multi-layer film filter or a
diffraction grating straightly transmitting only light having a
wavelength of .lamda..sub.1 and light having a wavelength of
.lamda..sub.2 in an annular region obtained by subtracting a region
of numerical aperture NA.sub.3 from a region of numerical aperture
NA.sub.1, to perform aperture limitation to light beams for CD
having a wavelength of .lamda..sub.3, a stable wavefront aberration
correction for CD can be performed.
[0271] Further, by forming a diffraction grating straightly
transmitting only light having a wavelength of .lamda..sub.1 in the
first annular region obtained by subtracting a region of numerical
aperture NA.sub.2 from a region of numerical aperture NA.sub.1, to
be used for an aperture-limiting element for light beams for DVD
having a wavelength of .lamda..sub.2, a stable wavefront aberration
correction for DVD can be performed.
[0272] The entire disclosures of Japanese Patent Application No.
2002-223085 filed on Jul. 31, 2002, Japanese Patent Application No.
2002-248835 filed on Aug. 28, 2002, Japanese Patent Application No.
2002-251911 filed on Aug. 29, 2002, Japanese Patent Application No.
2002-295731 filed on Oct. 9, 2002 and Japanese Patent Application
No. 2002-372435 filed on Dec. 24, 2002 including specifications,
claims, drawings and summaries are incorporated herein by reference
in their entireties.
* * * * *